W 


-  72- 


A   TEXTBOOK 

OF 

FIRE  ASSAYING 


BY 

EDWARD  E.  BUGBEE 

Assistant  Professor  of  Mining  Engineering  and  Metallurgy, 
Massachusetts  Institute  of  Technology 


NEW  YORK 

JOHN  WILEY  &  SONS,  INC. 

LONDON:   CHAPMAN  &  HALL,  LIMITED 
1922 


COPYRIGHT,  1922 

BY 
EDWARD  E.  BUGBEE 


TECHNICAL  COMPOSITION  CO. 
CAMBRIDGE,  MASS.,  U.  S.  A. 


PREFACE 

This  book  is  the  outgrowth  of  a  set  of  mimeograph  notes  pre- 
pared in  1911  and  intended  for  use  in  the  course  in  fire  assaying  at 
the  Massachusetts  Institute  of  Technology.  The  mimeograph 
notes  were  succeeded  by  a  book  of  150  pages  published  by  the 
author  in  1915.  The  present  volume  has  been  revised  and  en- 
larged and  is  offered  as  a  small  contribution  toward  the  scientific 
explanation  of  the  ancient  art  of  fire  assaying.  It  contains  some 
hitherto  unpublished  results  of  research,  as  well  as  considerable 
new  data  derived  from  a  careful  search  of  all  the  available  litera- 
ture, none  of  which  have  previously  appeared  in  book  form. 

Although  intended  primarily  as  a  college  textbook,  it  is  not  en- 
tirely elementary  in  character  and  it  is  hoped  that  it  will  be  found 
sufficiently  complete  and  fundamental  to  be  of  service  to  the  more 
mature  student  of  the  science.  Every  effort  has  been  made  to 
avoid  the  old  " cook-book"  method  of  presentation  so  common 
in  books  of  this  kind  and  to  give  the  underlying  scientific  reasons 
for  the  many  phenomena  which  occur,  as  well  as  the  rationale  of 
each  process  and  detail  of  manipulation. 

The  object  of  instruction  in  fire  assaying  should  not  be  merely 
the  training  of  students  to  obtain  results  of  a  certain  degree  of 
precision  by  blindly  following  some  set  procedure,  as  is  unfortu- 
nately too  often  the  case.  On  the  contrary,  their  attention  should 
be  focussed  on  the  physical  and  chemical  principles  which  govern 
the  various  operations.  If  they  truly  understand  the  reasons  for 
the  use  of  each  of  the  reagents  and  for  the  various  details  of 
technique,  they  will  not  have  to  hunt  over  the  pages  of  a  receipt 
book  when  confronted  by  an  ore  of  unfamiliar  constitution,  but 
will  be  able  to  make  up  their  own  assay  charges  and  outline  their 
own  details  of  manipulation. 

The  author  believes  that  a  course  in  fire  assaying  is  the  logical 
place  to  introduce  the  study  of  metallurgy.  The  study  of  gen- 
eral metallurgy,  which  is  abstract  and  uninteresting  by  itself, 
is  made  concrete  and  intensely  interesting  if  the  various  processes 
of  fire  assaying  are  used  to  illustrate  its  principles.  Most  of  the 

iii 

468460 


IV  PREFACE 

principles  of  metallurgy  are  utilized  in  one  stage  or  another  of  the 
fire  assay  and  if  taught  in  this  connection,  the  student's  interest  is 
awakened,  the  principles  are  understood  and  the  study  of  this 
branch  of  metallurgy  becomes  a  pleasure  and  not  a  burden.  With 
this  end  in  view,  emphasis  has  been  laid  on  those  metallurgical 
principles  which  are  of  importance  in  fire  assaying,  for  example, 
the  thermochemistry  of  the  metals  and  of  their  oxide  and  sul- 
phide compounds,  the  nature  and  physical  constants  of  slags,  the 
characteristics  of  refractories  and  fuels,  the  principles  of  ore 
sampling,  the  behavior  of  metallic  alloys  on  cooling  and  the  chem- 
ical reactions  of  oxidation  and  reduction. 

In  the  short  time  allowed  for  instruction  in  fire  assaying  in  the 
crowded  curricula  of  our  technical  schools,  the  time  factor  is  an 
important  consideration.  With  large  classes  and  a  limited  num- 
ber of  laboratory  instructors,  the  author's  experience  leads  him 
to  the  conclusion  that  it  is  inadvisable  to  rely  too  much  on  verbal 
instruction  in  the  classroom  and  laboratory,  particularly  during 
the  first  few  weeks  when  so  much  that  is  entirely  new  has  to 
be  mastered  before  any  real  progress  can  be  made.  Explicit  direc- 
tions are  given,  therefore,  for  the  first  analyses;  thus  saving  the 
student's  time  and  conserving  his  efforts  by  making  it  possible  for 
him  to  attack  the  subject  intelligently  and  without  any  unneces- 
sary delay.  As  the  work  progresses,  less  stress  is  laid  upon  de- 
tailed procedure  and  the  student  is  placed  more  upon  his  own  re- 
sources and  encouraged  to  work  out  his  own  assay  charges  from 
his  knowledge  of  fundamental  principles,  aided  by  a  study  of 
typical  examples. 

The  order  of  arrangement  of  laboratory  work  is  the  logical  one 
beginning  with  cupellation,  first  in  the  qualitative  and  then  in  the 
quantitative  way.  The  assay  of  lead  bullion  leads  naturally  to 
parting  for  the  determination  of  the  gold,  after  which  either  scori- 
fication  or  crucible  assaying  may  be  undertaken. 

When  available,  the  source  of  what  may  be  termed  "new  in- 
formation" has  been  acknowledged,  but  this  has  not  always  been 
possible  and  the  author  trusts  he  may  be  pardoned  for  any  serious 
omissions  in  this  particular.  Although  it  is  hoped  that  in  the 
present  book  all  of  the  errors  which  occurred  in  the  author's  edi- 
tion have  been  eliminated,  some  new  ones  may  have  crept  in  and 
the  author  will  esteem  it  a  favor  to  have  these  called  to  his  atten- 
tion. He  would  also  be  pleased  to  receive  any  suggestions  and 


PREFACE  V 

criticisms  which  might  be  embodied  in  a  subsequent  edition,  if 
such  should  be  required. 

To  the  many  friends  who  have  supplied  material  or  helped  in 
other  ways  the  writer  wishes  to  express  his  gratitude.  The  offi- 
cials of  the  Anaconda  Copper  Mining  Company  and  of  the  United 
States  Smelting,  Refining  and  Mining  Company  have  been  es- 
pecially helpful  in  this  way.  The  author  is  particularly  indebted 
to  Mr.  Rufus  C.  Reed  for  many  helpful  suggestions  and  for  read- 
ing the  type  script.  He  wishes  also  to  express  his  appreciation  of 
the  courtesy  of  the  Allis-Chalmers  Mfg.  Co.,  the  Braun  Corpora- 
tion, the  Denver  Fire  Clay  Co.,  the  Thompson  Balance  Co.,  and 
the  United  States  Bureau  of  Mines  for  furnishing  photographs 
and  electrotypes. 


CONTENTS 


CHAPTER  I 

PAGES 

ASSAY  REAGENTS  AND  FUSION  PRODUCTS  1-15 

Definitions.  Reagents.  Chemical  Reactions  of  Reagents. 
Fusion  Products. 

CHAPTER  II 

FURNACES  AND  FURNACE  ROOM  SUPPLIES  16-38 

Crucible  Furnaces.  Muffle  Furnaces.  Fuel.  Coal  Furnaces. 
Wood  Furnaces.  Coke  Furnaces.  Gasoline  Furnaces.  Gas 
Furnaces.  Fuel  Oil  Furnaces.  Furnace  Repairs.  Muffles. 
Crucibles.  Scorifiers.  Furnace  Tools. 

CHAPTER  III 

ORE  SAMPLING 39-70 

Definitions.  Methods.  Commercial  Considerations.  Prin- 
ciples of  Sampling.  Sampling  Practice.  Hand  Cutting.  Ma- 
chine Cutting.  Grab  Sampling.  Moisture  Sampling.  Du- 
plicate Sampling.  Finishing  the  Sample.  Size  of  Assay 
Pulp.  Sampling  Ore  Containing  Malleable  Minerals. 

CHAPTER  IV 

BALANCES  AND  WEIGHTS   71-88 

Flux  Balance.  Pulp  Balance.  Assay  Balance.  Theory  of 
Balance.  Directions  for  Use  of  Balance.  Weighing  by  Equal 
Swings.  Weighing  by  Method  of  Swings.  Weighing  by  No 
Deflection.  Weighing  by  Substitution.  Check  Weighing. 
Adjusting  and  Testing  Assay  Balance.  Weights.  Calibration 
of  Weights. 

CHAPTER  V 

CUPELLATION 89-117 

Bone  Ash.  Making  Cupels.  Description  of  Process.  Prac- 
tice in  Cupellation.  Assay  of  Lead  Bullion.  Loss  of  Silver 
in  Cupeling.  Loss  of  Gold  in  Cupeling.  Effect  of  Silver 
on  the  Loss  of  Gold  in  Cupeling.  Influence  of  Impurities  on  the 
Loss  of  Precious  Metals  during  Cupellation.  Rule  Governing 

vii 


yiii  CONTENTS 

PAGES 

Cupellation  Losses.  Indications  of  Metals  Present.  Indi- 
cations of  Rare  Metals.  Retention  of  Base  Metals.  Portland 
Cement  and  Magnesia  Cupels.  Color  Scale  of  Temperature. 

CHAPTER  VI 

PARTING 118-126 

General  Statement.  Parting  in  Porcelain  Capsules.  In- 
quartation.  Parting  in  Flasks.  Influence  of  Base  Metals  on 
Parting.  Indications  of  Presence  of  Rare  Metals.  Errors 
Resulting  from  Parting  Operations.  Testing  Nitric  Acid  for 
Impurities.  Testing  Wash  Water.  Testing  Silver  Foil  for 
Gold. 

CHAPTER  VII 

THE  SCORIFICATION  ASSAY 127-142 

General  Statement.  Solubility  of  Metallic  Oxides  in  Litharge. 
Heat  of  Formation  of  Metallic  Oxides.  Ignition  Temperature 
of  Metallic  Sulphides.  Assay  Procedure  for  Ores.  Chemical 
Reactions.  Indications  of  Metals  Present.  Assay  of  Gran- 
ulated Lead.  Scorification  Assay  of  Copper  Matte.  Losses 
in  Scorification.  Scorification  Charges  for  Different  Materials. 

CHAPTER  VIII 

THE  CRUCIBLE  ASSAY 143-195 

Theory  of  the  Crucible  Assay.  Classification  of  Ores.  Cru- 
cible Slags.  Classification  of  Silicates.  Action  of  Borax  in 
Slags.  Fluidity  of  Slags.  Acidic  and  Basic  Slags.  Mixed  Sili- 
cates. The  Lead  Button.  The  Cover.  Reduction  and  Oxi- 
dation. Reducing  Reactions.  Reducing  Power  of  Minerals. 
Oxidizing  Reactions.  Testing  Reagents.  Slags  for  Class  1 
Siliceous  Ores.  Slags  for  Class  1  Basic  Ores.  Assay  Procedure 
for  Class  1  Ores.  Assay  of  Class  2  Ores.  The  Niter  Assay. 
Slags  for  Pure  Ores.  Slags  for  Impure  Ores.  Conduct  of  the 
Fusion.  Physical  and  Chemical  Changes  Taking  Place  in 
Niter  Fusion.  Preliminary  Fusion.  Estimating  Reducing 
Power.  Calculation  of  Assay  Charge.  Procedure  for  the  Reg- 
ular Fusion.  The  Soda-Iron  Method.  Chemical  Reactions. 
The  Slag.  Procedure.  The  Roasting  Method.  Assay  of  Class 
3  Ores. 

CHAPTER  IX 

THE  ASSAY  OP  COMPLEX  ORES  AND  SPECIAL  METHODS 196-210 

Assay  of  Ores  Containing  Nickel  and  Cobalt.  Assay  of  Tel- 
luricle  Ores.  Assay  of  Ores  and  Products  High  in  Copper. 
Assay  of  Zinc-Box  Precipitate.  Assay  of  Antimonial  Ores. 


CONTENTS  ix 

PAGES 

Assay  of  Auriferous  Tinstone.     Corrected  Assays.     Assay  of 
Slag.     Assay  of  Cupels. 

CHAPTER  X 

THE  ASSAY  OF  BULLION 211-232 

Definitions.  Weights.  Sampling  Bullion.  Lead  Bullion. 
Copper  Bullion.  Dore  Bullion.  Gold  Bullion.  Assay  of  Lead 
Bullion.  Assay  of  Copper  Bullion.  Scorification  Method. 
Crucible  Method.  Nitric  Acid  Combination  Method.  Mercury- 
Sulphuric  Acid  Method.  Assay  of  Dore  Bullion.  United 
States  Mint  Assay  of  Gold  Bullion. 

CHAPTER  XI 

THE  ASSAY  OF  SOLUTIONS 233-239 

Evaporation  in  Lead  Tray.  Evaporation  with  Litharge. 
Precipitation  by  Zinc  and  Lead  Acetate.  Precipitation  as  Sul- 
phide. Precipitation  by  Cement  Copper.  Precipitation  by 
Silver  Nitrate.  Precipitation  by  a  Copper  Salt.  Electrolytic 
Precipitation.  Colorimetric  Method. 

CHAPTER  XII 

THE  LEAD  ASSAY 240-247 

General  Statement.  Lead  Ores.  Accuracy  and  Limitations 
of  Method.  Quantity  of  Ore  and  Reagents  Used.  Manipu- 
lation of  Assay.  Influence  of  Other  Metals.  Procedure  for 
Assay.  Assay  of  Slags.  Chemical  Reactions. 

INDEX..  .  249-254 


A  TEXTBOOK 
OF  FIRE  ASSAYING 


CHAPTER  I. 
ASSAY  REAGENTS  AND  FUSION   PRODUCTS. 

Assaying  is  a  branch  of  analytical  chemistry,  generally  defined 
as  the  quantitative  estimation  of  the  metals  in  ores  and  furnace 
products.  In  the  western  part  of  the  United  States,  the  term 
is  employed  to  include  the  determination  of  all  the  constituents, 
both  metallic  and  non-metallic,  of  ores  and  metallurgical  products. 

Fire  assaying  is  the  quantitative  determination  of  rnetals  in  ores 
and  metallurgical  products  by  means  of  heat  and  dry  reagents. 
This  involves  separating  the  metal  from  the  other  constituents  of 
the  ore  and  weighing  it  in  a  state  of  purity. 

An  ore  is  a  mineral-bearing  substance  from  which  a  metal, 
alloy  or  metallic  compound  can  be  extracted  at  a  profit.  The 
term  is  loosely  used  to  include  almost  any  inorganic  substance 
that  may  occur  in  nature.  An  ore  generally  consists  of  two  parts, 
^he  metalliferous  or  valuable  portion,  and  the  "  gangue  "  or  value- 
less portion.  Gangue  minerals  are  divided,  according  to  their 
chemical  composition,  into  two  classes,  acid  and  basic.  Silica 
is  a  type  of  the  former;  lime,  magnesia,  and  the  oxides  of  iron, 
manganese,  sodium  and  potassium  are  examples  of  the  latter. 

An  ore  may  be  acid,  basic  or  "  self -fluxing  "  according  to  the 
preponderance  of  one  or  the  other  group  of  slag-forming  gangue 
constituents.  A  self-fluxing  ore  is  one  which  contains  acid  and 
basic  material  in  the  right  proportion  to  form  a  slag. 

The  metallurgical  products  which  come  to  the  assayer  include 
bullion,  matte,  speiss.  drosses  and  crusts,  litharge,  flue-dust  and 
fume,  as  well  as  solutions  and  precipitates  resulting  from  hydro- 
metallurgical  operations. 

The  reagents  used  in  fire  assaying  may  be  classified  as  fluxes, 
acid,  basic  or  neutral,  and  as  oxidizing,  reducing,  sulphurizing  or 

1 


2  A   TEXTBOOK  OF  FIRE  ASSAYING 

desulphurizing  agents.  Some  reagents  have  only  one  property, 
as  for  instance  silica,  an  acid  flux,  others  have  several  different 
properties,  as  litharge,  a  basic  flux  but  also  an  oxidizing  and  de- 
sulphurizing agent. 

A  flux  is  something  which  converts  compounds  infusible  at 
a  certain  temperature  into  others  which  melt  at  this  temperature. 
For  instance,  quartz  by  itself  is  fusible  only  at  a  very  high  tem- 
perature, but  if  some  sodium  carbonate  is  added  to  the  pulverized 
quartz  it  can  be  fused  at  a  temperature  easily  obtained  in  the 
assay  furnace. 

The  student  should  remember  that  to  aid  in  the  fusion  of  an 
acid  substance,  a  basic  flux  such  as  litharge,  sodium  carbonate, 
limestone,  or  iron  oxide  should  be  added  while  for  a  basic  sub- 
stance an  acid  flux  such  as  silica  or  borax  should  be  used. 

A  reducing  agent  is  something  which  is  capable  of  causing  the 
separation  of  a  metal  from  the  substances  chemically  combined 
with  it  or  of  effecting  "  the  stepping  down  "  of  a  compound  from 
a  higher  to  a  lower  degree  of  oxidation. 

An  oxidizing  agent  is  one  which  gives  up  its  oxygen  readily. 

A  desulphurizing  agent  is  something  which  has  a  strong  affinity 
for  sulphur  and  which  is  therefore  capable  of  separating  it  from 
some  of  its  compounds. 

The  principal  reagents  used  in  assaying  follow: 

Silica,  Si02j  is  an  acid  reagent  and  the  strongest  one  available. 
It  combines  with  the  metal  oxides  to  form  silicates  which  are  the 
foundation  of  almost  all  slags.  It  is  used  as  a  flux  when  the  ore  is 
deficient  in  silica  and  serves  to  protect  the  crucibles  and  scorifiers 
from  the  corrosive  action  of  litharge.  Care  must  be  taken  to 
avoid  an  excess  of  silica,  as  too  much  of  it  will  cause  trouble  and 
losses  of  precious  metals  by  slagging  or  by  the  formation  of  matte. 
Silica  melts  at  about  1625°  C.  to  an  extremely  viscous  liquid. 
It  should  be  obtained  in  the  pulverized  form. 

The  fluxing  effect  of  silica  is  shown  in  the  accompanying  freez- 
ing-point curve*  of  the  lime-silica  series.  The  series  shows  three 
eutectics  and  two  compounds.  The  combination  having  the 
lowest  melting-point  is  the  eutectic  with  37  per  cent  of  CaO  which 
melts  at  1417°  C.  Another  eutectic  containing  54  per  cent  CaO 
melts  at  1430°  C.  Lying  between  these  is  the  compound  calcium 
bi-silicate,  corresponding  to  the  formula  CaSiO3,  which  fuses  at 
*  Day  and  Shepard  Am.  Jour.  Sc.  22,  p.  255. 


ASSAY  REAGENTS  AND  FUSION  PRODUCTS 


1512°  C.  A  second  compound,  corresponding  to  the  formula 
Ca2Si04,  the  calcium  singulo-silicate  melts  at  2080°  C.  A  cursory 
glance  at  this  curve  will  be  sufficient  to  suggest  the  desirability 
of  trying  to  make  approximately  a  bi-silicate  slag  when  assay- 
ing ores  which  contain  considerable  lime 


2100 
2000 
1900 

g/700 
^1600 
.1500 
1400 

%Si02  It 

A/. 

*OI5°C 

/ 
1 

/ 

1 

"^ 

;>s. 

^ 

J^""^ 

1 

/« 

I2°0 

14 

/v 

5 

*/45 

fo 

<2 

1         10       20       30       40        50       60        70        80       90       10 
10      90       80       70       60        50       40        30        20        10         0 

FIG.  1.  —  Freezing-point  curve  of  lime-silica  series. 

Glass  is  used  by  some  in  place  of  silica.  Ordinary  window-glass, 
a  silicate  of  lime  and  the  alkalies  with  the  silica  in  excess,  is  best. 
Its  acid  excess  is  always  doubtful  and  so  is  not  commonly  used. 
If  used,  a  blank  assay  should  be  run  on  each  new  lot  to  insure 
against  introducing  precious  metals  into  the  assay  in  this  way. 
Its  chief  advantage  is  that  5  or  10  grams  too  much  glass  will  or- 
dinarily do  no  harm  in  a  fusion  whereas  5  or  10  grams  of  silica  in 
excess  might  spoil  the  assay. 

Borax.  —  Na2B4O7,  10H2O,  is  an  active,  readily  fusible,  acid 
flux.  It  melts  in  its  own  water  of  crystallization,  beginning  at  the 
lowest  visible  red  heat,  and  becomes  anhydrous  at  a  full  red  heat. 
It  intumesces  in  fusing  and  on  account  of  this  behavior  may,  if 
used  in  large  amounts,  tend  to  force  part  of  the  charge  out  of 
the  crucible,  especially  if  not  thoroughly  mixed  with  the  charge. 


4 


A   TEXTBOOK  OF  FIRE  ASSAYING 


In  small  amounts,  however,  it  lowers  the  temperature  of  slag  for- 
mation and  promotes  a  quiet  and  orderly  fusion. 

Borax  is  often  used  as  a  cover  for  crucible  fusions.  When 
properly  used  it  is  believed  to  prevent  the  mechanical  loss  of  fine 
ore  which  frequently  results  when  a  large  volume  of  gas  escapes 
rapidly  at  a  temperature  below  that  of  incipient  fusion.  Borax,  con- 
taining, as  it  does,  47  per  cent  of  water,  loses  approximately  half  of 
its  weight  by  fusion,  and  consequently  when  used  as  an  acid  flux, 
approximately  twice  as  much  borax  as  borax-glass  is  required. 

Borax-Glass.  —  Na2B4O7,  is  made  by  fusing  borax  to  drive  off 
its  water  of  crystallization  and  then  cooling  and  crushing  the 
solidified  glassy  residue.  It  is  usually  purchased  in  the  powdered 
form  and  should  be  kept  in  air-tight  containers,  as  the  fine  material 
takes  on  moisture  from  the  air  and  tends  to  cake.  Under  ordinary 
conditions  it  behaves  like  a  true  glass,  having  no  definite  freezing- 
or  melting-point.  If,  however,  it  is  subjected  to  rapid  vibration 
when  cooling  it  may  be  induced  to  solidify  in  the  crystalline  form 
at  a  definite  temperature.  This  crystallized  borax-glass  melts 
at  742°  C.  If  not  subjected  to  vibration  it  remains  a  viscous 
fluid  even  below  a  visible  red.  Finely  divided  amorphous  borax- 
glass  begins  to  sinter  at  about  500°  C.  It  is  extremely  viscous  when 
melted,  even  when  heated  well  above  its  melting  temperature. 

Its  rational  formula,  Na2O,  2B2O3,  indicates  an  excess  of  acid, 
and  experiment  proves  this  to  be  present.  At  a  red  heat  it  be- 
comes a  strong  acid  and  dissolves  and  fluxes  practically  all  of  the 
metallic  oxides  both  acid  and  basic.  It  is  one  of  the  best  fluxes 
for  alumina. 

Five  borates  of  alkalies  and  alkaline  earths  are  recognized,  the 
chemical  classification  being  as  follows: 

TABLE  I. 

CLASSIFICATION  OF  BORATES. 


Name 

Oxygen  ratio 
Acid  to  base 

Formula 
R  =  bivalent  base 

Ortho-borate 

1  to  1 

3RO.B2O3 

Pyro-borate  .... 

1|  to  1 

2RO.B2O3 

Sesqui-borate  

2  to  1 

3RO.2B2O3 

Meta-borate  
Bi-borate 

3tol 
6  to  1 

RO.B2O3 
RO.2BoO3 

ASSAY  REAGENTS  AND  FUSION  PRODUCTS 


5 


According  to  the  metallurgical  classification,  i.e.,  the  ratio  of 
oxygen  in  acid  to  oxygen  in  base,  the  first  of  these  would  be  neutral 
and  the  others  acid.  Ditte*  studied  the  fused  borates  of  the 
alkaline  earths  and  classified  them  as  acid,  neutral  and  basic. 
He  called  the  meta-borate  neutral.  The  writer's  experiments  with 
alkaline  borates  show  that  the  meta-borate  is  decidedly  viscous 
when  fused  but  that  at  the  same  time  it  shows  a  strong  tendency 
to  crystallize  during  cooling.  The  pyro-borate,  when  fused,  was 
decidedly  fluid,  being  comparable  to  the  sub-silicate  of  soda. 
It  would  seem  proper,  therefore,  to  consider  the  sesqui-borate  as 
the  neutral  one  when  considered  from  this  standpoint. 


1100 
1056 

1000 

1 
^800 

"  700 
600 

%Na2B204L 

'\fcNa0SIOJ^ 

966 

<s. 

"^ 

^ 

\ 

_^  

^~^~ 

—  -  — 

' 

X 

N^ 

^ 

**^ 

—  814 

o 

1         /O        20        30        40        50       SO        70       50        90      100 
0      90        80        70        60        50        40       30        20        10         0 

FIG.  2. 


Freezing-point  curve  of  sodium  meta-borate  —  sodium  bi-silicate 
series. 


Fusing  as  it  does  at  a  low  temperature,  borax  helps  to  facilitate 
the  slagging  of  the  ore,  and  in  the  hydrous  or  anhydrous  condition 
is  used  in  almost  every  crucible  assay.  In  general,  it  may  be 
said  to  lower  very  appreciably  the  fusing-point  of  all  slags,  and  this, 
in  addition  to  the  fact  that  it  is  such  an  excellent  solvent  for  the 
metallic  oxides,  accounts  for  its  almost  universal  use  in  fire  assay- 
ing. The  borates  of  lead  and  the  alkalies  are  more  viscous  than 
the  corresponding  silicates.  This  viscous  effect  persists  far  below 
the  apparent  solidification-point  unless  the  slag  is  decidedly  basic. 

If  too  much  borax  is  used  in  the  assay  of  siliceous  ores  there 
*  Compt,  rend.  77,  p.  785,  p.  893  (1873). 


6  A   TEXTBOOK  OF  FIRE  ASSAYING 

results  a  very  tough,  glassy  or  stony  slag  which  holds  tenaciously 
to  the  lead  button.  This  is  probably  due  partly  to  the  effect 
of  borax  in  reducing  the  coefficient  of  expansion  of  the  slag  and 
partly  to  its  action  in  preventing  crystallization.  When  the  at- 
tempt is  made  to  separate  the  lead  and  slag,  a  film  of  lead  will 
often  adhere  to  the  slag  and  give  the  assayer  much  trouble. 

The  remedy  for  this  condition  is,  first,  to  reduce  the  quantity 
of  borax  used  and  then,  if  necessary,  to  increase  the  bases.  No 
more  than  5  or  10  grams  of  common  borax  or  its  equivalent  in 
borax-glass  should  be  used  per  assay-ton  of  siliceous  ore. 

The  melting-point  curve  of  the  sodium  meta-borate  —  sodium 
bi-silicate  series,  according  to  Van  Klooster*,  is  shown  in  Fig.  2. 
The  melting-point  of  sodium  bi-silicate  does  not  agree  exactly 
with  that  given  by  Niggi  but,  none  the  less,  the  diagram  serves 
to  illustrate  the  effect  which  borax  has  in  reducing  the  melting- 
point  of  assay  slags. 

The  eutectic  containing  56.5  per  cent  of  Na^SiOa  freezes  at 
814°  C. 

Sodium  bicarbonate,  NaHCO3,  is  still  used  by  some  assayers  as 
an  alkaline  flux,  mainly  because  of  its  cheapness  and  purity.  It 
is,  however,  decomposed  when  heated  to  276°  C.,  forming  the 
anhydrous  normal  carbonate  with  the  liberation  of  water  vapor 
and  carbon  dioxide.  The  large  volume  of  water  vapor  and  car- 
bon dioxide  released,  passing  up  through  the  charge  before  it  has 
softened,  is  bound  to  carry  off  more  or  less  of  the  fine  ore  and  thus 
contributes  to  the  so-called  "  dusting  "  loss. 

The  bicarbonate  contains  but  63.4  per  cent  of  NaaCOa  and 
therefore  when  it  is  used  as  a  substitute  for  the  normal  carbonate 
158  parts  are  required  for  each  100  parts  of  the  normal  carbonate. 
Because  of  the  above  serious  disadvantages  the  use  of  the  anhy- 
drous normal  carbonate  is  recommended  in  all  cases.  The  only 
advantage  which  can  now  be  claimed  for  the  bicarbonate  is  that 
it  does  not  deliquesce. 

Sodium  carbonate,  NaaCOs,  is  a  powerful  basic  flux  and  by  far 
the  cheapest  one  available  for  assay  purposes.  Owing  to  the  ease 
with  which  alkaline  sulphides  and  sulphates  are  formed  it  also 
acts  to  some  extent  as  a  desulphurizing  and  oxidizing  agent. 
Pure  anhydrous  sodium  carbonate  melts  at  852°  C.  When  molten 
it  is  very  fluid  and  can  hold  in  suspension  a  large  proportion  of 

*  Zeitschr.  anorg.  Chemie,  69,  p   122  (1910). 


ASSAY  REAGENTS  AND  FUSION  PRODUCTS 


finely  ground,  infusible  and  inactive  material  such  as  carbon  or 
bone-ash. 

The  commercial  normal  carbonate  of  this  country,  made  by  the 
Solvay  process  from  the  bicarbonate,  is  easily  obtained  in  a  pure 
state.  It  tends  to  absorb  water  from  the  air  and  is,  therefore, 
unsatisfactory  for  use  in  some  climates.  The  variety  known  by 
the  trade  as  58  per  cent  dense  soda-ash  has  been  found  particularly 
satisfactory  for  assay  purposes,  and  is  but  little  affected  by  at- 
mospheric moisture. 


1500 
1400 
1300 

1  1200 

1 

1  noo 

1000 
900 

-   800 

foCaSi03    t 
foNa2S/03IO 

FIG.  3.  - 

1502° 

/ 

X 

/ 

/ 

/ 

/- 

^-  

^ 

/ 

-^^ 

x 

/ 

"S 

<932' 

10       2.0       30       40        50       60       70       80       90      100 
0       90       80        70       60        50       40       30       20        10        0 

-Freezing-point  curve  of  calcium  bi-silicate  —  sodium  bi-silicate 

series. 

When  sodium  carbonate  is  heated  to  about  950°  C.,  it  undergoes 
a  slight  dissociation  with  the  consequent  evolution  of  a  small 
amount  of  carbon  dioxide.  Analysis  of  sodium  carbonate  which 
has  been  melted  shows  it  to  contain  about  0.4  per  cent  of  free 
alkali.  When  silica  is  added  to  the  fused  carbonate  this  free 
alkali  first  disappears  and  then  a  reaction  takes  place  between  the 
silica  and  sodium  carbonate  and  a  certain  amount  of  carbon  dioxide 
is  evolved.  The  amount  evolved  is  directly  proportional  to  the 
amount  of  silica  added  and  to  the  temperature.  Niggi*  showed 
that  the  system  Na^COs  —  SiO2,  for  a  constant  temperature  and 

*  Jour.  Am.  Chem.  Soc.  35,  pp.  1693-1727. 


8  A    TEXTBOOK  OF  FIRE  ASSAYING 

pressure  of  C02,  reaches  a  state  of  equilibrium,  which  condition 
may  be  expressed  by  the  equation: 

Na4Si04  +  CO2. 


He  found  that  in  order  to  displace  all  the  CO2,  at  least  one  mol  of 
Si02  for  each  mol  of  Na2O  must  be  added.  Combinations  less 
acid  than  the  bi-silicate  retain  CO2  indefinitely.  The  bi-silicate 
melts  at  about  1018°  C. 

The  fluxing  effect  of  sodium  carbonate  is  shown  in  the  ac- 
companying freezing-point  curve*  of  the  calcium  bi-silicate  — 
sodium  bi-silicate  series. 

Between  the  melting-point  of  sodium  bi-silicate,  1018°  C.  and 
that  of  calcium  bi-silicate,  1502°  C.,  Wallace  found  indications  of 
a  eutectic  containing  20  per  cent  CaSiO3  which  melted  at  932°  C. 
It  may  be  concluded  from  this  that  if  we  are  to  flux  limestone  with 
soda  and  silica  alone,  we  should  add  4  mols  of  Na2CO3  for  each 
mol  of  CaCO3,  or  roughly  60  grams  of  Na2CO3  for  J  A.T.  of  pure 
CaCO3,  together  with  sufficient  silica  for  a  bi-silicate.  The  ad- 
dition of  borax  will  materially  reduce  the  melting-temperature  of 
the  mixture. 

Potassium  carbonate,  K2CO3,  is  a  basic  flux,  similar  in  its  action 
to  sodium  carbonate.  It  melts  at  894°  C.  It  has  the  disadvan- 
tage of  being  more  expensive,  weight  for  weight,  than  sodium  car- 
bonate, and  because  of  its  greater  molecular  weight  more  of  it  is 
required  than  of  sodium  carbonate  to  produce  a  given  result. 

Niggif  showed  that  a  small  amount  of  silica  displaces  an  almost 
equivalent  amount  of  CO2  from  fused  potassium  carbonate,  and 
that  successive  additions  of  silica  displace  a  progressively  smaller 
quantity  of  CO2,  until  when  the  proportions  are  2  mols  of  Si02  to 
1  mol  of  K2O,  the  silica  displaces  only  half  the  equivalent  amount 
of  CO2,  at  which  condition  the  last  of  the  CO2  passes  off.  He  gives 
the  following  equation  as  expressing  the  conditions  of  equilib- 
rium: 

K2CO3  +  K2Si2O5  <=±  2K2SiO3  +  C02. 

WillorfJ  contends  that  diminution  of  the  partial  pressure  of 
CO2  causes  considerable  displacement  of  the  equilibrium  toward 
the  right-hand  side  of  the  equation.  With  this,  Niggi  does  not 

*  Zeitschr.  anorg.  Chemie,  63,  p.  1  (1909). 

t  loc.  cit. 

J  Zeitschr.  anorg.  Chemie,  39,  187  (1904). 


ASSAY  REAGENTS  AND  FUSION  PRODUCTS 


9 


agree  and  argues  that  the  influence  of  the  partial  pressure  of  CO2 
is  inconsiderable.  He  cites  experimental  data  as  well  as  theoretical 
grounds  for  this  belief. 

As  is  the  case  with  a  mixture  of  sodium  and  potassium  car- 
bonates, a  mixture  of  sodium  and  potassium  silicates  melts  at  a 
lower  temperature  than  either  one  alone,  and  for  this  reason 
the  mixture  is  used  whenever  it  is  desired  to  maintain  a  low 
temperature  during  the  assay.  The  lead  assay  is  an  example 
and  in  fact  is  now  about  the  only  case  in  which  it  is  still  cus- 
tomary to  use  potassium  carbonate  in  fire  assaying. 

Litharge,  PbO,  is  a  readily  fusible  basic  flux.  It  acts  also  as  an 
oxidizing  and  desulphurizing  agent  and  on  being  reduced  it  sup- 
plies the  lead  necessary  for  the  collection  of  the  gold  and  silver. 
It  melts  at  883°  C.,  and  contains  92.8  per  cenl  of  lead. 


900 


1*800 


^700 


76 


7/7 


740° 


766°, 


too 


90 


60 


80  70 

Equivalent  Percentage  PbO 

FIG.  4.  —  Freezing-point  curve  of  litharge-silica  series. 


50 


Mixtures  of  finely  pulverized  litharge  and  silica,  ranging  from 
6PbO.SiO2  to  PbO.SiO2,  begin  to  sinter  at  about  700°  C.  Ac- 
cording to  Mostowitch*  the  sub-silicate,  4PbO.SiO2,  is  completely 
liquefied  at  726°  ;  the  singulo-silicate,  2PbO,  Si02,  forms  a  viscous 
liquid  at  724°  but  does  not  flow  readily  until  heated  to  940°  C. 
The  bi-silicate,  PbO.SiO2,  melts  at  770°  and  eutectic  mixtures  both 
lower  and  higher  in  silica  fuse  at  lower  temperatures. 


*  Trans.  A.I.M.E.  56,  p.  744. 


10  A    TEXTBOOK  OF  FIRE  ASSAYING 

The  freezing-point  curve  of  part  of  the  PbO  —  SiO2  system,  ac- 
cording to  Hilpert-Nacken,*  is  shown  in  Fig.  4.  The  melting- 
points  of  compounds  shown  do1  not  agree  exactly  with  those  of 
Mostowitch.  Compared  with  sodium  bi-silicate,  which  melts 
at  1018°,  the  corresponding  lead  silicate  is  decidedly  more  fusible. 
This  explains  why  it  is  customary  to  provide  for  the  presence  of 
litharge  in  almost  all  assay  slags. 

Litharge  has  such  a  strong  affinity  for  silica  that  if  the  crucible 
charge  does  not  contain  enough  of  the  latter,  the  acid  material  of 
the  crucible  itself  will  be  attacked.  If  left  long  enough,  a  hole 
may  be  eaten  through  it. 

Litharge  readily  gives  up  its  oxygen  if  heated  with  carbon, 
hydrogen,  sulphur,  metallic  sulphides,  iron,  etc.  It  thus  acts  as 
an  oxidizing  and,  in  the  presence  of  sulphur,  as  a  desulphurizing 
agent.  Examples  of  these  reactions  are  shown  below: 

2PbO  +  C  =  CO2  +  2Pb  (oxidizing), 
3PbO  +  ZnS  =  ZnO  +  S02  +  3Pb  (desulphurizing 
and  oxidizing). 

The  liberated  lead  is  then  available  for  the  collection  of  the 
gold  and  silver. 

The  reaction  with  carbon  begins  below  500°  C.,  and  is  rapid  at 
600°.  Reduction  by  CO  starts  below  200°. 

Litharge  begins  to  volatilize  at  800°  C.  which  is  considerably 
below  its  melting-point. 

Lead  silicates  do  not  readily  give  up  their  lead  to  carbonaceous 
and  sulphurous  reducing  agents.  The  higher  the  proportion  of 
silica,  the  less  readily  is  the  silicate  broken  up.  In  order  that  all 
the  lead  may  be  extracted  it  must  first  be  set  free  by  the  use 
of  a  stronger  basic  flux.  Hofmanf  says,  "  metallic  iron  decom- 
poses all  fusible  lead  silicates  at  a  bright  red  heat,  provided  enough 
is  added  to  form  a  singulo-silicate." 

Ordinarily  commercial  litharge  contains  a  small  amount  of  silver, 
varying  from  0.2  ounce  to  1.0  ounce  or  over  per  ton.  A  prac- 
tically silver-free  variety  is  made  from  Missouri  lead  by  giving 
a  zinc  treatment,  as  for  the  Parkes  process,  and  then  cupeling. 
It  is  never  safe  to  assume,  however,  that  litharge  is  silver-free  until 

*  M6tallurgie  8,  p.  157  (1911). 

t  Metallurgy  of  Lead,  p.  38  (1918). 


ASSAY  REAGENTS  AND  FUSION  PRODUCTS  11 

it  has  been  proven  so  by  assay.     Each  new  lot  received  should 
therefore  be  carefully  mixed  to  make  it  uniform,  and  assayed. 

Assay  litharge  should  be  free  from  bismuth,  as  this  will  be 
reduced  during  the  fusion  and,  owing  to  its  slow  rate  of  oxidation, 
will  concentrate  in  the  lead  during  cupellation,  finally  giving 
irregular  silver  results. 

Lead  in  the  granulated  form,  test  lead,  is  used  in  the  scorifi- 
cation  assay  as  a  collector  of  the  precious  metals  and  as  a  flux. 
When  oxidized  by  the  air  of  the  muffle  it  becomes  a  basic  flux. 
Ordinary  test  lead  may  contain  more  or  less  silver  and  every  new 
lot  should  be  assayed  before  being  used. 

Test  lead  may  be  made  by  pouring  molten  lead,  just  above  its 
freezing-point,  into  a  wooden  box  and  shaking  it  violently  in  a 
horizontal  direction  just  as  it  becomes  pasty  and  continuing  until 
it  becomes  solid.  The  fine  material  is  sifted  out,  the  coarse  is 
remelted. 

Lead  in  the  form  of  foil  is  used  in  the  fire  assay  of  gold,  silver 
and  lead  bullion.  Lead  melts  at  326°  C.  Like  litharge  it  should 
be  free  from  bismuth. 

Argols  is  a  reducing  agent  and  basic  flux.  It  is  a  crude  bitar- 
trate  of  potassium  obtained  from  wine  barrels,  and  is  one  of  the 
best  reducing  agents. 

Cream  of  tartar,  KHC4H4O6,  is  refined  bitartrate  of  potassium. 
Being  free  from  sulphur  it  is  used  as  a  reducing  agent  in  the  copper 
assay.  Both  argols  and  cream  of  tartar  break  up  on  heating  as 
follows : 

2KHC4H4O6  +  heat  =  K2O  +  5H2O  +  6CO  +  2C. 
The  K2O  thus  liberated  is  available  as  a  flux. 

Charcoal,  sugar,  flour  etc.  are  also  reducing  agents  because  of 
the  carbon  that  they  contain.  Flour  is  very  commonly  used  in 
flux  mixtures  and  is  satisfactory  in  every  respect. 

Iron  is  a  desulphurizing  and  reducing  agent.  When  it  is  heated 
with  the  sulphides  of  lead,  silver,  mercury,  bismuth  and  antimony 
the  sulphides  are  decomposed,  yielding  a  more  or  less  pure  metal 
and  iron  sulphide.  Copper,  nickel  and  cobalt  sulphides  are 
partly  reduced  by  iron,  as  would  be  expected  from  a  study  of  the 
heats  of  formation  of  the  same. 

Iron  also  reduces  most  of  these  metals  and  some  others  from  their 
oxide  combinations,  as  for  example : 

PbO  +  Fe  =  Pb  +  FeO, 


12  A   TEXTBOOK  OF  FIRE  ASSAYING 

the  iron  oxide  formed  acts  as  a  basic  flux.     Iron  decomposes  all 
fusible  lead  silicates  by  replacing  the  lead,  thus: 

xPbO.SiO2  +  xFe  =  xFeO.SiO2  +  xPb. 

It  should  therefore  always  be  used  in  the  lead  assay. 

It  is  used  in  the  form  of  spikes  or  nails,  and  sometimes,  es- 
pecially in  Europe,  an  iron  crucible  is  employed. 

Potassium  nitrate,  KNO3,  commonly  known  as  niter,  is  a  power- 
ful oxidizing  agent.  It  melts  at  339°  C.  and  fuses  at  a  low  tem- 
perature without  alteration,  but  at  a  higher  temperature  it  breaks 
up,  giving  off  oxygen,  which  oxidizes  sulphur  and  many  of  the 
metals,  notably  lead  and  copper. 

It  is  used  in  the  fire  assay  especially  to  oxidize  sulphides,  ar- 
senides, antimonides,  etc. 

If  fused  alone  it  is  stable  until  a  temperature  of  530°  C.  is 
reached,  when  it  begins  to  decompose,  giving  off  oxygen.  When 
it  is  fused  with  charcoal,  the  two  begin  to  react  at  about  440°  C. 
The  reaction  between  niter  and  carbon,  according  to  Roscoe  and 
Schoerleman,  is  as  follows  : 

4KNO3  +  5C  =  2K2C03  +  3CO2  +  2N2 

According  to  the  same  authority,  sulphur  and  niter  react  as 
follows  : 

2KNO3  +  2S  =  K2SO4  +  SO2  +  N2. 

Niter  begins  to  react  with  silica  at  about  450°  C.,*  probably 
according  to  the  following  reaction: 

4KNO3  +  2Si02  =  2K2SiO3  +  5O2  +  2N2. 

In  a  charge  containing  a  large  excess  of  soda  and  litharge  the 
reaction  with  pyrite  is  as  follows  : 

6KNO3  +  2FeS2  +  Na^CO,  = 

Fe2O3  +  3K2S04  +  NaaSO*  +  CO2  +  3N2. 


Many  assayers  object  to  the  use  of  niter  because  of  its  oxidizing 
effect  on  silver.  Large  amounts  of  niter  cause  boiling  of  the 
crucible  charge  -and  necessitate  careful  heating  to  prevent  loss. 
It  is  found  to  give  less  trouble  when  the  crucible  is  uniformly 
heated,  as  in  the  muffle,  than  when  the  charge  begins  to  melt  first 
at  the  bottom,  as  in  the  pot-furnace. 

*  Fulton,  A  Manual  of  Fire  Assaying,  p.  59 


ASSAY  REAGENTS  AND  FUSION  PRODUCTS 


13 


Potassium  cyanide,  KCN,  is  a  powerful  reducing  and  desulphur- 
izing agent.  It  combines  with  oxygen,  forming  potassium  cyanate, 

thus: 

PbO  +  KCN  =  Pb  +  KCNO  (reducing  action), 

and  also  with  sulphur,  forming  sulphocyanide,  as  follows: 
PbS  +  KCN  =  Pb  +  KSCN. 

It  is  sometimes  used  in  the  lead  assay  and  usually  in  the  tin  and 
bismuth  assays.  It  is  extremely  poisonous,  and  should  be  handled 
with  great  care.  It  fuses  at  526°  C. 

Salt,  NaCl,  is  used  as  a  cover  to  exclude  the  air,  and  to  wash 
the  sides  of  the  crucible  and  prevent  small  particles  of  lead  from 
adhering  thereto.  It  melts  at  819°  C. 

It  does  not  enter  the  slag,  but  floats  on  top  of  it.  It  is  often 
colored  by  the  different  metallic  oxides  of  the  charge  and  sometimes 
helps  to  distinguish  assays  which  have  become  mixed  in  pouring. 

TABLE  II. 

ASSAY  REAGENTS. 


Name 

Formula 

Properties  in  order  of  their  importance 

Silica 

SiO2 

Acid  flux 

Glass 

xNa2O.yCaO.zSiO2 

Acid  flux 

Borax 

Na2B4O7.10H2O 

Acid  flux 

Borax-glass 

Na2B4O7 

Acid  flux 

Sodium  bicarbonate 

NaHCOa 

Basic  flux,  desulphurizing 

Sodium  carbonate 

Na2C03 

Basic  flux,  desulphurizing 

Potassium  carbonate 

K2C03 

Basic  flux,  desulphurizing 

Litharge 

PbO 

Basic    flux,    desulphurizing, 

oxidizing 

Potassium  nitrate 

KNO3 

Oxidizing,  desulphurizing 

Argols 

KHC4H4O6  +  C 

Reducing  agent,  basic  flux 

Cream  of  tartar 

KHC4H4O6 

Reducing  agent,  basic  flux 

Flour 

Reducing  agent 

Charcoal 

C 

Reducing  agent 

Lead 

Pb 

Collecting  agent 

Iron 

Fe 

Desulphurizing  and  reducing 

agent 

Potassium  cyanide 

KCN 

Reducing  and  desulphurizing 

agent 

Salt 

NaCl 

Cover  and  wash 

Fluorspar 

CaF2 

Neutral  flux 

Cryolite 

AlNa3F6 

Neutral  flux 

Fluorspar,  CaF2,  is  occasionally  used  as  a  flux  in  fire  assaying. 
Its  melting-point  is  1378°  C.  and  it  would,  therefore,  seem  to  be  of 
doubtful  value  in  fire  assay  fusions  which  seldom  exceed  1200°  C. 


14  A    TEXTBOOK  OF  FIRE  ASSAYING 

When  melted  it  is  very  fluid  and  assists  in  liquefying  the  charge, 
although  it  is  inert  and  does  not  ordinarily  enter  into  chemical 
combination  with  the  other  constituents  of  the  charge.  Kar- 
andeeff*  shows  a  melting-point  curve  of  CaF2  —  CaSiO3  series 
with  a  eutectic,  containing  54  molecular  per  cent  of  CaSi03,  which 
fuses  at  1128°  C. 

Cryolite,  AlNa3F6,  is  a  powerful  flux,  commonly  used  in  the 
manufacture  of  enamels  and  occasionally  in  the  melting  of  bullion. 
It  may  sometimes  be  useful  in  fire  assaying.  Cryolite  melts  at 
about  1000°  C.  and  has  the  property  of  dissolving  alumina.  It 
increases  the  coefficient  of  expansion  of  the  slag. 

Fusion  Products.  —  Every  gold,  silver  or  lead  assay  fusion,  if 
the  charge  is  properly  proportioned  and  manipulated,  should  show 
two  products,  a  lead  button  and,  above  it,  a  slag.  Two  undesir- 
able products,  matte  and  speiss  are  occasionally  also  obtained. 
When  a  cover  of  salt  is  used,  or  if  niter  is  used  in  the  assay,  a 
third  product  will  be  found  on  top  of  the  solidified  slag.  In  the 
first  case  this  is  almost  entirely  sodium  chloride,  in  the  latter  case 
it  is  a  mixture  of  the  sulphates  of  sodium  and  potassium. 

THE  LEAD  BUTTON  should  be  bright,  soft  and  malleable  and 
should  separate  easily  from  the  slag.  It  should  contain  practi- 
cally all  of  the  gold  and  silver  which  were  in  the  ore  taken  for  the 
assay. 

SLAG  is  a  fusible  compound  of  earthy  or  metallic  oxides  and 
silica  or  other  acid  constituents.  The  'slags  made  in  fire  assaying 
are  usually  silicates  or  borates  of  the  metallic  oxides  contained  in 
the  ore  and  fluxes  used. 

Slags  should  be  homogeneous  and  free  from  particles  of  unde- 
composed  ore.  A  good  slag  is  usually  more  or  less  glassy  and 
brittle.  When  poured,  the  slag  should  be  thin  and  fluid  and  free 
from  shots  of  lead.  If  too  acid,  it  will  be  quite  viscous  and  stringy, 
and  the  last  drops  will  form  a  thread  in  pouring.  If  too  basic, 
it  will  be  lumpy  and  break  off  short  in  pouring.  When  cold,  the 
neutral  or  acid  slag  is  glassy  and  brittle,  the  basic  one  is  dull  and 
stony-looking. 

Slags  in  the  molten  state  are  usually  solutions,  but  in  rare 
cases  they  may  be  chemical  compounds.  In  the  solid  state  they 
are  usually  either  solid  solutions  or  eutectic  mixtures;  occasion- 
ally they  may  be  chemical  compounds. 

*  Zeitschr.  anorg.  Chemie,  68,  p.  188  (1910). 


ASSAY  REAGENTS  AND  FUSION  PRODUCTS  15 

MATTE  is  an  artificial  sulphide  of  one  or  more  of  the  metals, 
formed  in  the  dry  way.  In  assaying  it  is  most  often  encountered 
in  the  niter  fusion  of  sulphide  ores  when  the  charge  is  too  acid.  It 
is  found  lying  just  above  the  lead  button.  It  is  usually  blue-gray 
in  color,  approaching  galena  in  composition  and  very  brittle. 
It  may  form  a  layer  of  considerable  thickness,  or  may  appear 
simply  as  a  granular  coating  on  the  upper  surface  of  the  lead  but- 
ton. This  matte  always  carries  some  of  the  gold  and  silver  and, 
as  it  is  brittle,  it  is  usually  broken  off  and  lost  in  the  slag  in  the 
cleaning  of  the  lead  button.  The  student  should  examine  the 
lead  button  as  soon  as  it  is  broken  from  the  slag  and  if  any  matte 
is  found,  he  may  be  certain  that  his  charge  or  furnace  manipulations 
are  wrong. 

SPEISS  is  an  artificial,  metallic  arsenide  or  antimonide  formed 
in  smelting  operations.  As  obtained  in  the  fire  assay,  it  is  usually 
an  arsenide  of  iron  approaching  the  composition  of  Fe5As.  Oc- 
casionally the  iron  may  be  replaced  by  nickel  or  cobalt.  The  anti- 
mony speiss  is  very  rare.  In  assaying,  speiss  is  obtained  when  the 
iron  method  is  used  on  ores  containing  arsenic.  It  is  a  hard, 
fairly  tough,  tin-white  substance  found  directly  on  top  of  the 
lead  and  usually  adhering  tenaciously  to  it. 

If  only  a  small  amount  of  arsenic  is  present  in  the  ore,  the  speiss 
will  appear  as  a  little  button  lying  on  top  of  the  lead;  if  much 
arsenic  is  present,  the  speiss  will  form  a  layer  entirely  covering  the 
lead.  It  carries  some  gold  and  silver.  If  only  a  gram  or  so  in 
weight,  it  may  be  put  into  the  cupel  with  the  lead  and  will  be  oxi- 
dized there,  giving  up  its  precious  metal  values  to  the  lead  bath. 
A  large  amount  of  speiss  is  very  hard  to  deal  with  as  it  is  difficult 
to  scorify.  The  best  way  is  to  assay  again,  by  some  other  method. 


CHAPTER  II. 
FURNACES  AND   FURNACE  ROOM   SUPPLIES. 

Furnaces  for  assaying  may  be  divided  into  the  two  following 
classes : 

1.  Crucible  or  Pot-Furnaces.  —  These  are  furnaces  used  solely 
for  melting  purposes,  in  which  the  crucible  is  in  direct  contact  with 
the  fuel  or  flame  and  the  contents,  therefore,  more  or  less  subject 
to  the  action  of  the  products  of  combustion. 

2.  Muffle  Furnaces.  —  These  are  furnaces  in  which  the  charge 
to  be  heated  is  in  a  space,  the  muffle,  apart  from  the  fuel  or  prod- 
ucts of  combustion.     The  muffle  is  a  semi-cylindrical  receptacle 
of  fire-clay  or  other  refractory  material,  set  horizontally  and  so 
arranged  that  the  fuel  or  products  of  combustion  pass  around  and 
under  it.     Thus,  the  material  to  be  heated  is  entirely  separated 
from  the  products  of  combustion. 

As  muffle  furnaces  may  be  used  for  melting  purposes  as  well  as 
for  scorification  and  cupellation,  many  assayers  in  America  use 
this  type  of  furnace  exclusively,  especially  in  connection  with  soft 
coal  fuel.  The  advantages  of  muffle  furnaces  for  melting  are  the 
greater  ease  and  saving  of  time  in  charging  and  pouring,  the  better 
control  of  temperature  and  the  better  distribution  of  heat  for 
melting  purposes.  Crucibles  also  seem  to  stand  more  heats  in  a 
muffle  furnace  than  they  will  in  pot-furnaces,  probably  on  account 
of  the  slower  and  more  uniform  heating. 

Pot-furnaces  have  the  advantage  of  size,  so  that  in  dealing 
with  low-grade  ores,  for  instance,  a  larger  charge  and  crucible 
may  be  used  than  in  muffle  furnaces  of  the  ordinary  size.  A 
higher  temperature  may  be  obtained  in  pot-furnaces  than  in 
muffles  and  this,  occasionally,  is  an  advantage  of  the  pot-furnace. 

The  furnaces  themselves  are  made  of  fire-brick  or  fire-clay  tile 
and  may  be  set  in  an  iron  jacket  or  surrounded  by  common  red 
brick.  Fire-brick  is  best  laid  in  a  mortar  made  from  a  mixture 
of  2  parts  ground  fire-brick  and  1  part  fire-clay.  Sometimes  a 
small  amount  of  Portland  cement  is  added.  In  any  event  the 

16 


FURNACES  AND  FURNACE  ROOM  SUPPLIES  17 

brick  and 'tiles  should  be  thoroughly  wet  before  the  mortar  is 
applied.  Finally,  as  little  mortar  as  possible  should  be  used,  since 
the  bricks  are  much  harder  than  the  solidified  mortar. 

Assay  furnaces  are  made  to  burn  practically  all  kinds  of  gaseous, 
liquid  and  solid  fuels.  Those  most  commonly  used  are  natural  and 
artificial  gas,  gasoline,  kerosene,  fuel-oil,  wood,  charcoal,  coke, 
bituminous  and  anthracite,  coal. 

Gas  is  the  cleanest,  most  easily  controlled,  most  efficient  in 
combustion  and,  except  in  the  case  of  a  natural  supply,  the  most 
expensive  fuel.  When  gas  is  used  for  firing,  a  blower  is  usually 
required  to  supply  air  under  a  low  pressure. 

Oil  is  nearly  as  clean  and  as  convenient  to  use  as  gas,  the  effi- 
ciency of  combustion  is  high  and  in  localities  near  the  oil-fields  it 
may  be  very  cheaply  obtained.  The  calorific  power  of  the  hydro- 
carbon fuel-oils  is  high,  about  50  per  cent  more  than  the  best  coals, 
which  makes  them  particularly  suited  for  use  in  isolated  localities 
where  freight  charges  are  high.  Gasoline  is  forced  under  pressure 
through  a  heated  burner  where  it  is  vaporized,  and  the  gas  in- 
jected into  the  furnace  carries  with  it  a  sufficient  supply  of  air  for 
combustion.  Fuel-oil  requires  steam  or  air  under  pressure  to  aid 
in  atomizing  the  oil,  preliminary  to  proper  combustion.  Gasoline 
and  kerosene  both  have  a  heating  value  of  about  21,000  B.t.u. 
per  pound,  crude  petroleum  about  18,500. 

Solid  fuels  are  usually  the  cheapest  and  are  therefore  more  ex- 
tensively used  than  any  of  the  others.  In  isolated  districts  where 
coal  or  coke  is  not  available,  wood  is  occasionally  used  as  fuel  in 
assay  furnaces.  For  this  purpose  the  wood  should  be  felled  in 
winter  and  thoroughly  air-dried  for  at  least  six  months  or  longer, 
according  to  the  climate.  The  air-dried  wood  will  still  retain 
from  20  to  25  per  cent  of  water  and  in  this  condition  has  a  heating 
value  of  about  6000  B.t.u.  per  pound.  Charcoal  is  seldom  used 
in  this  country  for  assay  purposes  on  account  of  the  abundance 
of  other  fuels. 

Bituminous  coal  is  the  most  satisfactory  solid  fuel  for  muffle- 
furnace  firing  and  coke  for  pot-furnaces.  Good  soft  coal  has  a 
calorific  power  of  about  14,500  B.t.u.  per  pound.  It  should  be 
low  in  sulphur  and  the  ash  must  not  be  too  readily  fusible.  Coke 
should  be  hard,  strong  and  low  in  sulphur,  and  the  ash  should  be 
infusible  at  the  temperature  of  the  furnace.  That  is  to  say,  it 
should  be  high  in  silica  and  alumina  and  low  in  iron,  calcium, 


18  A   TEXTBOOK  OF  FIRE  ASSAYING 

magnesium  and  the  alkalies,  to  prevent  clinkering  of  the  walls  of 
the  furnace. 

Gas  and  Oil  vs.  Solid  Fuel.  —  Gaseous  and  liquid  fuels  have 
many  advantages  over  solid  fuels  for  assay  purposes.  Some  of 
these  advantages  are  as  follows: 

1.  The  fire  is  kindled  in  an  instant  and  the  furnace  may  be 
quickly  heated  to  the  desired  temperature  for  work. 

2.  The  temperature  is  readily  controlled  and  may  be  quickly 
varied  to  suit  the  requirements  of  the  work. 

3.  A  high  efficiency  of  combustion  is  possible  in  properly  de- 
signed furnaces,  and  as  soon  as  the  work  is  completed  the  fuel 
supply  may  be  shut  off  and  fuel  consumption  stopped. 

4.  The  avoidance  of  labor  in  firing  gives  the  assayer  more  time 
for  other  duties. 

5.  The  cleanliness  in  operation,  due  to  absence  of  solid  fuel  and 
ash,  is  obviously  a  great  advantage  in  any  analytical  laboratory. 

On  account  of  the  expense,  however,  coal  is  much  more  gener- 
ally used  than  either  oil  or  gas.  It  is  easy  to  make  a  comparison 
of  the  costs  of  any  of  the  fuels  by  considering  the  heat  units.  For 
instance,  with  soft  coal  at  $10  per  ton  and  gasoline  at  25  cents 
per  gallon,  1  cent  invested  in  soft  coal  may  be  said  to  buy  2  X 
14,500  =  29,000  B.t.u.  and  the  same  amount  invested  in  gasoline 
to  buy  approximately  ^  X  6.0  X  21,000  =  5040  B.t.u.  That  is 
to  say,  the  gasoline  is  over  six  times  as  expensive  as  the  coal  on 
the  basis  of  heat  units,  and  for  steady  running  this  may  be  taken 
to  be  approximately  correct.  However,  for  a  small  amount  of 
work,  a  gasoline  furnace  may  be  cheaper  to  run  even  with  the  cost 
of  fuel  as  above  assumed,  for  the  small  furnace  is  quickly  heated 
and  as  soon  as  the  work  is  completed  the  oil  supply  may  be  shut  off 
and  the  expense  stopped,  while  a  coal  furnace  takes  much  longer 
to  heat  and  then  must  be  allowed  to  burn  out  after  the  work  is 
completed. 

Coal  Furnaces.  —  This  type  of  furnace  is  used  in  many  of  the 
large  custom  and  smelter  assay  offices  in  this  country. 

The  furnace  may  be  built  either  with  a  tile  or  fire-brick  lining. 
The  tile  lining  is  more  easily  set  up,  but  whether  or  not  it  is  as 
durable  as  a  properly  constructed  fire-brick  lining  is  open  to 
question.  The  outside  of  the  furnace  is  usually  laid  up  with  com- 
mon hard-burned  red  brick.  If  the  furnace  is  to  be  lined  with 
fire-brick  several  rows  of  "  headers  "  should  be  left  to  hold  the 


FURNACES  AND  FURNACE  ROOM  SUPPLIES 


19 


lining  securely  in  place.     The  furnace  is  held  together  with  angle- 
irons,  stays  and  tie-rods. 


FIG.  5.— Twin  double-muffle  soft  coal  furnace. 

A  great  improvement  in  the  construction  of  these  furnaces  may 
be  made  by  introducing  a  single  course  of  insulating  brick  between 
the  fire-brick  and  the  red  brick.  The  use  of  these  brick  permits 


20 


TEXTBOOK  OF  FIRE  ASSAYING 


a  quicker  heating  of  the  furnace,  affords  a  considerable  economy  in 
fuel  and  provides  a  much  more  comfortable  working  place,  be- 
cause of  a  large  reduction  in  the  heat  losses  due  to  radiation. 


FIG.  6. — Longitudinal  section  of  double-muffle  soft  coal  furnace. 

In  Figs.  5,  6  and  7  are  shown  front  elevation,  longitudinal  and 
transverse    sections    of    twin    double-muffle    soft    coal    furnaces 


FURNACES  AND  FURNACE  ROOM  SUPPLIES 


21 


using  NN  muffles.     In  Fig.  5  is  also  shown  an  iron-topped  pour- 
ing table  with  slagging-anvils  made  of  sections  of  steel  rails.     The 


FIG.  7.— Transverse  section  of  double-muffle  soft  coal  furnace. 

operator  stands  between  the  table  and  the  furnace  when  working 
at  the  furnace,  and  on  the  other  side  of  the  table  when  hammering 
out  his  lead  buttons.  Insulating  brick  is  used  in  the  construe- 


22  A   TEXTBOOK  OF  FIRE  ASSAYING 

tion  of  these  furnaces,  and  the  system  of  bonding  the  wall  with 
"  headers  "  is  shown  in  the  sections. 

In  the  furnace,  as  ordinarily  constructed,  the  muffles  are  sup- 
ported by  "  jamb  "  bricks  projecting  from  the  sides.  When  these 
are  used,  it  is  well  to  leave  a  hole  or  loose  brick  on  the  outside  of 
the  furnace,  to  facilitate  the  removal  of  the  stubs  when  these 
bricks  become  broken  off  and  the  ends  slagged  in.  Fulton  recom- 
mends using  long  tiles  which  meet  in  the  center,  thus  giving 
better  support  for  the  muffle.  He  claims  a  prolonged  life  for 
the  muffle  with  this  arrangement.  The  writer  has  found  the 
Scotch  Gartcraig  brick  to  outlast  3  or  4  best  American  fire-brick 
for  muffle  supports.  Another  method  of  supporting  muffles  in 
furnaces  of  this  type  is  by  the  use  of  iron  pipes  or  castings  which 
extend  directly  across  the  furnace  and  through  which  cooling 
water  is  circulated. 

These  furnaces  occupy  a  floor  space  of  approximately  3J  by 
4  feet.  They  are  built  in  a  variety  of  sizes;  those  taking  NN, 
QQ  and  UU  muffles  are  the  sizes  most  commonly  used.  The 
NN  muffle  is  10J  by  19  by  6J  inches  outside,  the  QQ  is  12J  by 
19  by  7f  and  the  UU  is  14  by  19  by  7J  inches  outside. 

Each  NN  muffle  will  hold  twelve  20-gram,  or  eight  30- 
gram  crucibles,  allowing  in  each  case  for  a  row  of  empty  crucibles 
in  front  to  act  as  warmers,  while  the  QQ  muffle  will  hold  fifteen  20- 
gram,  or  twelve  30-gram  crucibles,  also  allowing  for  a  row  of 
empty  crucibles  in  front. 

The  furnaces  are  best  arranged  to  be  fired  from  the  rear,  al- 
though they  may  be  arranged  to  be  fired  from  the  front  or  sides. 
The  flue  makes  off  from  near  the  front  of  the  furnace,  thus  tend- 
ing to  heat  the  muffle  uniformly  throughout  its  entire  length.  It 
should  be  from  one-sixth  to  one-eighth  the  grate  area. 

The  stack  for  one  of  the  furnaces  will  need  to  be  at  least  20  feet 
high  and  possibly  higher,  depending  largely  on  the  character  of 
the  coal.  It  should  not  be  built  directly  on  the  furnace  but  may 
be  placed  directly  over  the  furnace  if  supported  by  arches  and  cast- 
iron  columns,  or  it  may  be  put  to  one  side  of  the  furnace  and  in 
this  case  will  extend  down  to  the  ground.  When  the  stack  is 
supported  independently  of  the  furnace  it  allows  the  furnace  to 
expand  and  contract  with  less  danger  of  cracking  and  also  permits 
of  tearing  down  and  rebuilding  the  furnace  without  interfering 
with  the  stack. 


FURNACES  AND  FURNACE  ROOM  SUPPLIES  23 

With  long-flame  coal  these  furnaces  are  best  fired  with  a  rather 
thin  bed  of  fuel,  say  6  inches.  The  sequence  of  firing  will  con- 
sist, first,  of  running  the  slice  bar  along  the  entire  length  of  the 
grate  in  one  or  two  places  and  lifting  up  the  fire  to  break  up  any 
large  cakes  and  thus  allow  free  passage  of  air  through  the  fire, 
second,  of  pushing  the  well  coked  coal  forward  with  the  hoe  and, 
third,  of  adding  1  or  2  shovels  of  fresh  coal  near  the  firing  door. 
As  this  coal  is  heated  it  begins  to  coke  and  the  gas  given  off  passes 
over  the  white-hot  coal  of  the  fire  and  is  there  mixed  with  heated 
air.  This  results  in  a  free  draft  and  good  volume  of  hot  flame. 
If  instead  of  being  added  near  the  firing  door  the  fresh  coal  is 
spread  all  over  the  fire  it  will  quickly  cake  and  tend  to  smother 
the  fire  by  shutting  off  the  draft. 

It  is  not  necessary  to  use  the  slice  bar  every  time,  but  only 
when  the  fire  is  tightly  caked  or  after  a  long  run  when  the  grate 
is  covered  with  clinkers. 

The  temperature  of  the  muffle  may  be  regulated  at  will  by  ma- 
nipulating the  draft-  and  firing-doors.  For  instance,  after  a  batch 
of  cupels  have  started,  the  draft  may  be  closed  and  the  firing- 
door  opened,  to  admit  cold  air  above  the  fire  and  quickly  cool  the 
muffles  to  any  required  degree. 

Soft  coal  furnaces  have  the  advantage  of  simplicity  and  low 
initial  cost.  They  burn  from  40  to  50  pounds  of  good  bituminous 
coal  an  hour. 

Wood  Furnaces.  —  Wood-burning  furnaces  are  made  with 
single  and  double  muffles  and  are  much  like  the  soft  coal  furnaces 
except  that  a  larger  firebox  and  grate  are  used.  Wood  is  usually 
sawed  in  2-foot  lengths  and  with  dry  wood  the  muffle  may  be 
easily  heated  sufficiently  for  assaying.  Hard  wood  is  much  to 
be  preferred  as  it  does  not  burn  out  as  rapidly,  but  almost  any 
kind  of  dry  wood  may  be  used. 

The  large  firebox  and  the  grate,  which  is  set  about  8  inches 
below  the  bottom  of  the  fire-door  are  the  principal  distinguishing 
characteristics  of  a  wood-burning  assay  furnace. 

Coke  Furnaces.  —  Coke  is  still  used  to  a  considerable  extent  in 
pot-furnaces,  but  for  muffle-furnace  fuel  it  is  fast  falling  into 
disuse,  at  least  in  this  country. 

Compared  with  the  soft-coal  muffle  furnace,  the  coke  furnace 
has  the  advantage  that  it  can  be  more  quickly  heated  to  a  cupel- 
ing temperature  and  that  it  requires  less  frequent  stoking.  On 


24 


A   TEXTBOOK  OF  FIRE  ASSAYING 


the  other  hand  it  is  harder  to  regulate  the  temperature,  espe- 
cially to  cool  it  off  quickly  when  cupeling;  the  stoking  is  harder 
work  and  in  most  localities  the  fuel  cost  per  assay  is  higher. 

The  great  advantage  of  the  coke  pot-furnace  is  the  very  high 
temperature  which  may  be  obtained  and  the  fact  that,  even 
though  the  crucibles  boil  over  or  eat  through,  no  harm  is  done 
to  the  furnace.  Coke  furnaces  should  be  supplied  with  an  espe- 
cially good  quality  of  fuel.  If  the  ash  tends  to  melt,  the  walls 
quickly  become  covered  with  clinkers  and  are  bound  to  be  more 
or  less  damaged  when  these  are  removed. 


FIG.  8. — Gasoline  furnace  outfit. 

Gasoline  Furnaces.  —  A  gasoline  furnace  outfit  consists  of  a 
furnace,  which  may  be  either  a  muffle,  crucible  or  combination  of 
the  two,  a  burner  with  piping,  etc.,  and  a  gasoline  tank  with 
pump.  The  tank  for  a  small  assay  office,  is  an  ordinary  tinned- 
steel  pressure  tank  equipped  with  a  hand  pump,  pressure-gage 


FURNACES  AND  FURNACE  ROOM  SUPPLIES  25 

and  the  necessary  piping  connections.  These  range  from  2  to  15 
gallons  capacity. 

A  complete  gasoline  furnace  outfit  is  shown  in  Fig.  8.  This  is  a 
combination  crucible  and  muffle  furnace  made  in  several  sizes 
ranging  in  capacity  from  6F  or  4G  crucibles  to  10F  or  6G  crucibles. 
In  the  illustration  the  crucible  compartment  is  shown  open  al- 
though, of  course,  when  actually  in  use,  it  is  closed  with  special 
fire-clay  covers.  The  muffle  is  situated  directly  above  the  cru- 
cible chamber.  The  advantage  of  this  type  of  furnace  is  that 
fusions  may  be  started  within  fifteen  minutes  after  the  heat  is 
turned  on,  and  while  the  fusions  are  in  progress  the  muffle  is 
heating.  At  the  end  of  two  rounds  of  fusions,  the  muffle  is  hot 
enough  for  cupellation. 

The  burners  are  usually  constructed  of  special  bronze  alloys 
capable  of  withstanding  oxidation  at  high  temperatures.  They 
consist  of  a  filtering  chamber  for  purifying  the  gasoline,  a  gener- 
ating chamber  where  the  gasoline  is  vaporized,  a  generating  pan 
and  valve  for  the  initial  heating  of  the  burner,  a  spraying  nozzle 
and  valve  through  which  the  gasoline  vapor  is  injected  into  the 
furnace  and  a  mixing  chamber  where  the  proper  amount  of  air  for 
combustion  is  mixed  with  the  gas.  From  the  filter  the  gasoline 
passes  around  the  interior  of  the  burner  face,  the  generating 
chamber,  where  it  is  heated  by  the  heat  radiated  from  the  furnace 
and  vaporized,  so  that  once  the  furnace  is  under  way  the  genera- 
ting burner  may  be  shut  off.  Gasoline  is  supplied  to  the  burner 
under  a  pressure  of  from  20  to  50  pounds  per  square  inch. 

The  great  object  to  be  sought  and  one  of  the  hardest  to  attain 
in  any  gasoline  furnace  is  an  even  distribution  of  heat.  Another 
objectionable  feature  in  many  gas  and  gasoline  furnaces  is  the  poor 
draft  through  the  muffle.  Owing  to  the  fact  that  the  pressure  in- 
side the  furnace  is  slightly  greater  than  that  of  the  atmos- 
phere there  is  a  great  tendency  for  the  products  of  combustion  to 
work  back  through  the  hole  in  the  rear  of  the  muffle,  thus  to  a 
large  extent  excluding  the  air  and  unduly  prolonging  cupellation 
or  scorification. 

In  operating  a  gasoline  burner  care  should  be  taken  to  see  that 
combustion  takes  place  only  in  the  furnace.  All  burners  have 
more  or  less  tendency  to  back-fire,  that  is  for  the  flame  to  jump 
back  and  remain  in  the  mixing  chamber.  If  this  is  allowed  to 
continue,  the  burner  gets  so  hot  that  the  metal  oxidizes  and  then 


26  A   TEXTBOOK  OF  FIRE  ASSAYING 

it  is  only  a  matter  of  a  short  time  before  it  is  entirely  destroyed. 
Every  furnace  should  be  provided  with  a  shut-off  valve  between 
the  burner  and  the  gasoline  tank.  When  it  is  desired  to  shut  off 
the  furnace,  close  this  valve,  letting  the  burner  continue  as  long 
as  any  pressure  is  left  and  never  entirely  close  the  burner  valves. 
The  valve  stem  or  needle  is  of  steel  and  the  seat  is  of  bronze,  and 
owing  to  the  different  rates  of  expansion  of  these  metals,  the  valve 
is  injured  if  these  are  left  in  close  contact  when  the  burner  is  cool- 
ing. This  precaution  is  especially  to  be  observed  when  the  burner 
is  provided  with  the  ordinary  needle  valve,  as  when  this  opening 
is  once  enlarged  the  efficiency  of  the  burner  is  destroyed. 

Gas  Furnaces.  —  Gas  furnaces  are  used  in  some  assay  offices, 
especially  where  ?  natural  gas  supply  is  available.  Where  artifi- 
cial gas  has  to  be  used  this  type  of  furnace  proves  decidedly  ex- 
pensive, if  used  for  any  considerable  amount  of  work.  As  the 
gas  is  not  usually  under  sufficient  pressure  to  carry  in  its  own 
supply  of  air  for  combustion,  these  furnaces  are  customarily 
supplied  with  air  from  a  blower,  which  adds  to  the  expense  and 
difficulty  of  the  furnace  operation. 

Fuel-Oil  Furnaces.  —  When  a  cheap  oil  supply  is  available, 
oil  is  an  ideal  fuel  for  assay  furnaces.  Fuel-oil  and  kerosene  can- 
not be  vaporized  in  the  burner  as  they  deposit  carbon  when  heated 
and  thus  clog  the  passages.  Consequently,  to  ensure  complete 
combustion,  the  oil  must  be  thrown  into  the  furnace  in  as  fine  a 
state  of  mechanical  subdivision  as  possible.  This  is  accomplished 
by  atomizing  the  oil  with  a  jet  of  air  or  steam. 

The  air  for  atomizing  the  oil  may  be  supplied,  (1),  by  a  high- 
speed motor-driven  fan  giving  a  large  volume  of  air  at  a  pressure 
of  from  6  to  14  ounces;  (2),  by  a  positive  pressure  blower  giving 
a  pressure  of  from  1  to  5  pounds;  or  (3),  directly  from  a  compres- 
sor. In  the  last  case,  the  air  is  reduced  to  any  desired  pressure 
by  a  suitable  regulating  valve. 

The  burner  used  must  be  designed  to  operate  properly  with 
air  at  the  available  pressure.  Therefore,  there  are  low-,  medium- 
and  high-pressure  burners.  The  high-pressure  systems  are  noisy 
and  therefore  objectionable  from  this  standpoint,  as  well  as  be- 
cause of  the  large  amount  of  power  required.  The  low-pressure 
burner,  operating  usually  at  6  or  8  ounces  air  pressure  makes  very 
little  noise  and  requires  a  comparatively  small  amount  of  power. 
It  is  said,  moreover,  to  use  less  oil  and  to  cause  less  damage 


FURNACES  AND  FURNACE  ROOM  SUPPLIES 


27 


to  the  furnace,  and  is  therefore  most  commonly  used.  A  section 
of  a  low-pressure  oil  burner  is  shown  in  Fig.  9.  This  burner 
is  adjustable  for  oil  and  air  so  that  a  wide  range  of  tempera- 
ture variation  is  available.  The  oil  is  introduced  through  the 
central  channel  and  the  quantity  admitted  is  regulated  by  a 
needle  valve.  The  oil  channel  terminates  in  an  enlarged  orifice 
through  which  the  oil  is  discharged  in  a  thin,  circular  film.  It 
is  caught  by  a  rotating  air  blast  and  discharged  from  the  nozzle 


AIR  ALWAYS 
AT  SAME 
PRESSURE 


FIG.  9. — Low  pressure  oil  burner,  sectional  view. 

as  a  fine  mist.  Air  for  atomizing  passes  through  the  cone  and 
is  given  a  whirling  motion  by  fins  which  project  from  it.  Extra 
air  for  combustion  passes  in  around  the  outside  of  the  cone, 
which  is  operated  from  the  side  by  means  of  a  rack  and  pinion, 
and  may  be  completely  shut  off  by  moving  the  cone  out  as  far  as 
it  will  go. 

Any  fuel-oil  lighter  than  18°  Baume  at  60°  F.  may  be  used  in 
these  burners  with  gravity  feed  and  only  a  few  feet  of  head.  A 
heavier  oil  may  be  used  if  heated,  although  in  this  case  a  pressure 
feed  may  be  desirable.  The  oil  consumption  for  assay  furnaces 
runs  from  \\  to  2|  gallons  per  hour,  dependent  on  the  size  of 
furnace  and  grade  of  oil. 


28  A    TEXTBOOK  OF  FIRE  ASSAYING 

The  muffle  type  of  furnace  is  commonly  used  both  for  fusions 
and  cupellations.  The  furnace  proper  may  be  considered  to 
consist  of  three  parts;  a  combustion  chamber  where  the  oil  is 
ignited,  a  muffle  chamber  which  contains  the  muffle  and  where 
combustion  is  completed,  and  the  damper  block  which  contains 
the  dampers  for  controlling  the  atmosphere  in  the  muffle  and  the 
flow  of  gases  through  the  furnace.  The  combustion  chamber 
serves  to  protect  the  muffle  from  the  intense  direct  heat  of  the 
flame  and  is  lined  with  removable  tiles.  Between  it  and  the 
muffle  is  a  heavy  fire-clay  plate  which  serves  as  a  support  for 
the  muffle  and  protects  it  from  the  flame.  By  adjusting  the 
dampers,  either  an  oxidizing  or  reducing  atmosphere  may  be 
obtained  in  the  muffle. 

Care  of  Muffles.  —  Muffles  are  expensive,  and  care  should  be 
taken  to  make  them  last  as  long  as  possible.  They  are  subject  to 
injury  by  corrosion  due  to  basic  reagents,  principally  litharge, 
and  by  cracking,  due  to  sudden  changes  of  temperature.  Care 
should  be  taken,  particularly  in  the  case  of  oil  furnaces,  to  bring 
the  heat  up  slowly  so  that  all  parts  of  the  furnace  may  become 
heated  gradually.  To  prevent  injury  by  corrosion  try  to  avoid  ac- 
cidental spilling,  and  so  proportion  the  size  of  crucibles  to  the 
charges  that  boiling  over  is  impossible.  Cupels  should  weigh 
20  per  cent  more  than  the  button  to  be  cupeled  in  order  to  pre- 
vent litharge  from  running  through  and  on  to  the  muffle  floor. 
Cracking  due  to  changes  of  temperature  is  much  more  rapid  when 
the  inside  of  the  muffle  is  glazed.  This  is  due  to  the  different 
rates  of  expansion  of  the  glazed  and  unglazed  parts. 

When  any  lead  or  slag  is  spilled  in  the  muffle,  or  a  fusion  is 
found  to  have  eaten  through  its  container,  the  muffle  must  be 
quickly  scraped  out  and  the  spot  well  covered  with  bone-ash. 
The  bone-ash  absorbs  the  litharge  and  forms  a  thick  paste  with 
the  slag  so  that  it  can  be  easily  cleaned  out  with  a  scraper.  It  is 
well  to  keep  a  thin  layer  of  bone-ash  in  the  muffle  at  all  times. 

When  not  in  use  the  drafts  and  muffle  doors  should  be  kept 
closed,  and  at  the  end  of  the  day  the  furnace  should  be  allowed 
to  cool  down  slowly. 

Furnace  Repairs.  —  Fire-clay  usually  forms  the  basis  of  mor- 
tars used  in  furnace  construction  and  repairs,  as  lime  mortar 
and  hydraulic  cement  are  not  suitable  for  use  with  masonry  ex- 
posed to  high  temperatures.  Fire-clay  is  a  clay  containing  only 


FUMNACES  AND  FURNACE  ROOM  SUPPLIES  29 

very  small  amounts  of  iron,  lime,  magnesia  and  the  alkali  oxides. 
It  forms  a  more  or  less  plastic  and  sticky  mortar;  on  heating  it 
loses  its  moisture  and  plasticity  and  the  mortar  hardens. 

All  clays  shrink  more  or  less  on  drying  and  burning,  and  to 
prevent  this  as  far  as  possible,  as  well  as  to  make  the  mortar 
strong,  a  certain  amount  of  crushed  fire-brick  or  sand  should  be 
added.  Crushed  fire-brick  is  better  than  sand  owing  to  its  porous 
and  irregular  shaped  grains,  as  these  give  a  better  mixture  with 
the  clay  and  a  stronger  cement. 

A  good  mortar  for  general  use  around  assay  furnaces-  is  made 
with  a  mixture  of  2  parts  fire-brick  ground  through  12-mesh  and 
1  part  fire-clay.  A  small  amount  of  Portland  cement  or  mold- 
ing clay,  say  not  over  J  part,  will  cause  the  mixture  to  adhere 
better  and  make  the  mortar  harder  when  set.  For  work  at  very 
high  temperatures  the  Portland  cement  must  be  omitted  as  it 
acts  as  a  flux  for  the  other  materials  and  causes  the  whole  to 
melt. 

All  mortars  should  be  made  up  dry  and  thoroughly  mixed  be- 
fore the  required  amount  of  water  is  added.  The  water  should 
be  thoroughly  mixed  in  and  the  mortar  should  be  sticky  and  of 
the  right  consistency.  It  is  well  to  mix  the  mortar  several  hours 
before  using.  When  bricks  are  being  laid  or  repairs  made  about 
a  furnace,  the  bricks  and  brickwork  should  be  thoroughly  wet 
before  the  mortar  is  applied,  as  otherwise  the  bricks  absorb  so 
much  water  that  the  mortar  does  not  form  a  good  bond  with  them. 

In  laying  fire-bricks,  as  little  mortar  as  possible  should  be  used 
as  the  bricks  are  always  harder  than  even  the  best  of  mortar. 
The  mortar  should  be  made  to  fill  every  crevice.  The  best  way 
to  attain  this  is  to  put  an  extra  amount  of  fairly  thin  mortar  on 
the  wet  brick  and  then  drive  or  force  it  firmly  into  place,  allow- 
ing the  excess  mortar  to  squeeze  out. 

The  ash  from  many  coals  is  quite  readily  fusible  and  results  in 
the  formation  of  clinkers  and  accretions  on  the  sides  of  the  furnace, 
especially  just  above  the  grate.  When  the  furnace  is  cold  these 
adhere  very  tenaciously  to  the  walls  of  the  furnace  and  when 
they  are  broken  off,  pieces  of  the  brick  are  removed  with  them. 
To  remove  these  accretions  with  the  least  possible  damage  to  the 
furnace  cut  them  off  with  a  chisel  bar  just  after  a  hot  fire  has  been 
drawn. 

In  putting  in  a  new  muffle,  first  remove  the  old  one  with  the 


30  A    TEXTBOOK  OF  FIRE  ASSAYING 

mortar  that  held  it,  also  any  clinkers  which  would  interfere  with 
the  working  of  the  furnace.  Patch  the  lining  of  the  furnace  if 
necessary  and  see  that  the  bricks  or  other  supports  for  the  muffle 
are  in  place  and  in  good  condition.  After  trying  the  muffle  to 
see  that  it  rests  properly  on  the  supports,  remove  it,  sponge  over 
the  brickwork  where  the  mortar  is  to  come  in  contact  with  it, 
place  some  rather  thick  mortar  on  each  of  the  supports  and  re- 
place the  muffle.  See  that  it  rests  evenly  on  the  different  sup- 
ports and  on  the  front  wall  of  the  furnace.  The  muffle  should  be 
level  or  slope  slightly  toward  the  front  end.  Fill  up  the  space 
between  the  muffle  and  the  front  wall  of  the  furnace  with  some 
rather  thick  mortar,  working  from  both  inside  and  outside  of 
the  furnace.  This  outside  joint  should  be  finished  up  neatly 
with  the  aid  of  a  trowel.  It  is  best  to  allow  the  furnace  to  dry 
for  a  day  or  two  if  possible,  but  if  necessary  it  may  be  used  as 
soon  as  finished  by  heating  up  slowly. 

For  patching  the  linings  of  furnaces  use  the  mixture  recom- 
mended for  general  use  or  try  the  following  which  is  recommended 
by  Lodge.  Fire-brick  through  12-mesh  7  parts;  Portland  cement 
2  parts,  fire-clay  1  part.  Put  this  on  as  dry  as  possible  and  it 
will  make  a  patch  almost  as  hard  as  the  original  brick. 

Cracked  and  broken  muffles  may  be  made  to  last  much  longer 
if  patched  with  one  of  the  above-mentioned  mixtures. 

Metallurgical  Clay  Goods. 

Under  the  caption,  "  Metallurgical  Clay  Goods,  "  are  included 
muffles,  crucibles,  scorifiers,  roasting-dishes,  annealing  cups  etc. 
These  embrace  many  of  the  most  important  utensils  of  the  assayer 
and  upon  their  good  properties  much  of  his  success  depends. 
Fire-clay  is  the  only  material  which  answers  the  double  purpose 
of  satisfactory  service  and  inexpensive  construction.  Refrac- 
tory clay  or  fire-clay,  as  it  is  commonly  called,  is  a  clay  which  will 
stand  exposure  to  a  high  temperature  without  melting  or  becom- 
ing, in  a  sensible  degree,  soft  or  plastic. 

All  clays  contract  both  upon  drying  and  upon  burning  and  this 
leads  to  more  or  less  warping  and  cracking  of  the  finished  product. 
To  prevent  this  shrinkage  as  far  as  possible  and  also  to  add  strength 
to  the  finished  article  it  is  customary  to  add  a  certain  amount  of 
sand  or  well-burned  clay  to  the  mixture.  Burned  clay  is  usually 
preferred  to  sand  for  this  purpose,  not  only  because  its  rough  por- 


FURNACES  AND  FURNACE  ROOM  SUPPLIES  31 

ous  grains  give  a  better  bond  with  the  fire-clay  and  make  a  stronger 
cement,  but  also  because  it  makes  an  article  which  is  less  readily 
corroded  by  assay  slags  and  fusion  products.  The  intermix- 
ture of  coarse  grains  of  burned  clay  also  helps  in  that  it  makes 
a  product  better  able  to  withstand  sudden  changes  in  tempera- 
ture. 

The  exact  proportions  of  raw  and  burned  clay  used  by  any 
manufacturer  are  carefully  guarded  trade  secrets  and  depend, 
of  course,  very  much  on  the  clay  used  as  well  as  on  the  article  to 
be  manufactured.  The  larger  the  article  the  more  care  must  be 
taken  to  prevent  warping  and  cracking.  Usually  however,  the 
proportion  of  raw  to  burned  clay  will  lie  between  the  limits  of 
1  to  1  and  1  to  2. 

Muffles.  —  Muffles  may  be  made  of  a  variety  of  materials, 
but  for  assay  purposes  fire-clay  muffles  are  used  exclusively.  They 
are  made  in  a  great  variety  of  sizes  and  shapes.  However,  when 
crucible  fusions  are  to  be  made  in  the  muffle,  a  nearly  rectangular 
cross-section  is  preferred,  as  this  gives  a  muffle  of  almost  uniform 
height  without  any  appreciable  waste  space. 

Muffles,  as  well  as  other  fire-clay  ware,  should  be  stored  in  a 
warm,  dry  place  and  should  be  heated  and  cooled  slowly  and  uni- 
formly. The  life  of  a  muffle  is  also  much  influenced  by  the  way 
it  is  supported. 

Crucibles.  —  Assay  crucibles  are  made  either  of  a  mixture  of 
raw  and  burned  clay  or  of  a  mixture  of  sand  and  clay,  the  first 
being  known  as  clay  or  fluxing  crucibles  and  the  second  as  sand 
crucibles.  The  raw  clay  is  finely  ground,  mixed  with  the  right 
proportion  of  coarser  particles  of  sand  or  burned  clay  and  water, 
and  the  whole  well  kneaded  and  compressed  in  molds  of  the  proper 
shape. 

Good  crucibles  should  have  the  following  properties: 

1.  Ability  to  withstand  a  high  temperature  without  softening. 

2.  Strength  to  stand  handling  and  shipping  without  breaking. 

3.  Ability   to   stand   sudden   changes   of   temperature   without 

cracking. 

4.  Ability  to  withstand  the  chemical  action  of  the  substances 

fused  in  them. 

5.  Impermeability  to  the  substances  fused  in  them  and  to  the 

products  of  combustion. 
Of  course  it  is  impossible  to  get  any  one  crucible  which  will 


32  A   TEXTBOOK  OF  FIRE  ASSAYING 

possess  all  of  the  above  good  properties  to  a  high  degree.  For 
instance  if  a  crucible  is  to  be  made  as  nearly  impermeable  as  possi- 
ble, it  will  be  made  of  very  fine-grained  material  and  tightly  com- 
pressed. Such  a  crucible,  however,  will  not  stand  handling  or 
sudden  changes  of  temperature  as  well  as  one  made  with  a  skele- 
ton of  coarser  material.  Furthermore  the  manner  and  tempera- 
ture of  burning  has  much  to  do  with  the  ability  of  crucibles  to 
stand  handling  and  shipping.  A  fairly  hard-burned  crucible  will 
be  stronger  and  less  likely  to  be  broken  in  handling,  but  on  the 
other  hand  it  will  not  stand  sudden  changes  of  temperature  as 
well  as  a  soft-burned  crucible.  Crucibles  made  of  clay  contain- 
ing little  uncombined  silica  and  of  burned  clay  of  the  same  nature 
will  stand  a  high  temperature  and  chemical  corrosion  much  better 
than  those  made  of  sand  and  clay  or  of  clay  containing  much  free 
silica. 

Crucibles  are  tested  for  resistance  to  chemical  corrosion  by 
actual  service  and  also  by  fusing  litharge  in  them  and  noting  the 
time  it  takes  to  eat  through.  To  make  a  test  of  this  sort  which  is 
of  any  value,  care  must  be  taken  to  see  that  the  temperature, 
the  quantity  of  litharge  and  all  other  conditions  are  the  same  for 
the  crucibles  being  tested.  A  crucible  may  be  tested  for  its 
permeability  to  liquids  by  filling  it  with  water  and  noting  the 
time  it  takes  before  it  becomes  moist  on  the  outside. 

Crucibles  come  in  a  great  variety  of  shapes  and  sizes.  Those 
most  commonly  used  for  assaying  may  be  classified  into  two 
groups  as  follows : 

Pot-Furnace  Crucibles.  —  These  are  comparatively  slim,  heavy 
walled  crucibles  with  practically  no  limit  as  to  height.  The  base 
is  small,  so  that  they  may  be  forced  down  into  the  fuel  and  for 
this  reason  they  are  easily  tipped  over  and  are  not  suitable  for 
muffle  work.  The  sizes  most  used  are  the  E,  F,  G,  H,  J  and  K. 
Crucibles  of  the  same  designation  but  made  by  different  manu- 
facturers vary  considerably  in  capacity.  The  approximate  ca- 
pacity of  some  of  the  pot-furnace  crucibles  is  shown  in  the  follow- 
ing table: 


FURNACES  AND  FURNACE  ROOM  SUPPLIES 


33 


TABLE  III. 
CAPACITIES  OF  POT-FURNACE  CRUCIBLES. 


Crucible  designation 

E 

F 

G 

H 

I 

j 

K 

1  Battersea 

180  c.c. 

210  c.c. 

300  c.c. 

420  c.c. 

600  c  c 

750  c  c 

2  Denver  

180  c.c. 

240  c.c. 

400  c.c. 

530  c.c. 

685  c.c. 

950  c.c. 

1  Made  by  the  Morgan  Crucible  Co.,  London,  England. 

2  Made  by  the  Denver  Fire  Clay  Co.,  Denver,  Colorado. 

Muffle  Crucibles.  —  These  are  made  with  a  broader  base  so 
that  they  may  stand  securely  on  the  floor  of  the  muffle,  and  are 
usually  not  more  than  4  inches  high.  Muffle  crucibles  are  desig- 
nated by  gram  capacity,  the  10-,  15-,  20-  and  30-gram  sizes  being 
most  frequently  used.  The  numbers  are  intended  to  indicate 
the  grams  of  ore-charge  which  the  crucibles  will  take.  They  are 
usually  generously  proportioned,  so  that  often  an  assay-ton  of  ore 
(29.166  grams)  may  be  treated  in  a  20-gram  crucible.  The  ap- 
proximate capacity  of  the  more  important  muffle  crucibles  is 
shown  in  the  following  table: 

TABLE  IV. 

CAPACITY  OF  MUFFLE  FURNACE  CRUCIBLES. 


Crucible  designation 

5  gm. 

10  gm. 

12  gm. 

15  gm. 

20  gm. 

30  gm. 

Denver  
Battersea  

70  c.c. 
70  c.c. 

100  c.c. 
100  c.c. 

140  c.c. 

160  c.c. 
135  c.c. 

190  c.c. 
190  c.c. 

260  c.c. 
260  c.c. 

Scorifiers.  —  These  are  shallow  fire-clay  dishes  used  in  the 
scorification  assay  of  gold  and  silver  ores.  They  should  be 
smooth  on  the  inside,  dense  and  impermeable  to  lead  and  slag 
and  should  be  composed  so  as  to  withstand,  as  much  as  possible, 
the  corrosive  action  of  litharge.  Scorifiers  are  designated  by 
their  outside  diameters.  Of  the  large  number  of  sizes  made, 
the  following  are  the  most  commonly  used :  2\  inches,  2 J  inches, 
2f  inches,  3  inches,  3J  inches.  The  Bartlett  scorifier  is  shal- 
lower than  the  regular  one  and  was  designed  for  the  treatment 
of  heavy  sulphide  ores  containing  considerable  metallic  im- 


34  A   TEXTBOOK  OF  FIRE  ASSAYING 

purities.  Scorifiers  should  be  made  of  clay  containing  a  mini- 
mum of  uncombined  silica,  as  the  scorifier  slags  are  usually  very 
basic.  Particularly  when  they  contain  copper,  they  attpck  the 
silica  of  a  scorifier  with  avidity,  and  one  with  a  siliceous  skeleton 
may  become  perforated  and  allow  its  contents  to  escape  to  the 
floor  of  the  muffle,  thus  spoiling  the  assay  and  injuring  the  muffle. 

FURNACE  TOOLS. 

The  more  important  furnace  tools  consist  of  crucible,  scorifier, 
cupel  and  annealing  cup  tongs,  cupel  rakes  and  shovels,  muffle 
scrapers  and  spatulas  and  the  various  pouring  molds,  cupel  and 
annealing  cup  trays,  hammers,  slagging  forceps,  anvils  etc. 


FIG.  10. — Crucible  tongs  for  use  in  muffle. 


FIG.  11. — Crucible  tongs  for  use  in  coke  pot-furnace. 


FIG.  12. — Crucible  tongs  for  use  in  gasoline  melting-furnace. 

Crucible  Tongs.  —  Two  types  of  crucible  tongs  are  in  common 
use,  those  which  grasp  the  body  of  the  crucible  and  those  which 
grip  the  top  edge  of  the  crucible  inside  and  out.  In  Fig.  10  is 
shown  a  pair  of  crucible  tongs  of  the  first  type,  suited  for,  use  in 
a  laboratory  where  the  fusions  are  made  in  the  muffle.  Thirty 
inches  is  a  convenient  length  for  these  tongs.  Figure  11  illus- 
trates a  good  strong  pair  of  the  second  type  of  tongs,  especially 
'suited  for  coke  pot-furnace  work.  These  may  be  made  some- 
what shorter,  say  26  inches  long.  Figure  12  shows  a  lighter  con- 


FURNACES  AND  FURNACE  ROOM  SUPPLIES 


35 


U 


struction  of  the  second  type  for  use  in  gasoline  melting-furnaces 
and  in  muffle  work.  These  should  be  about  30  inches  long.  A 
combination  of  these  two  types  of  tongs  is  listed  by  most  supply 
houses,  but  is  of  little  practical  use  as  it  cannot  be  used  in  a  muffle 
full  of  crucibles  to  grasp  the  body 
of  the  crucible,  owing  to  its  shape, 
and  neither  is  it  as  satisfactory  as 
the  one  illustrated  in  Fig.  12  for 
use  in  gasoline  melting-furnaces. 
Another  convenient  tool  for  crucible 
furnace  work  is  shown  in  Fig.  13. 
This  is  known  as  a  charging  fork. 
It  consists  of  a  fork-shaped  piece 
of  steel,  which  fits  the  crucible  about 
midway,  mounted  on  the  end  of  a 
long  rod.  This  is  used  principally 
for  charging  and  occasionally  for 
pouring;  46  to  48  inches  is  a  con- 
venient length. 

Scorifier  Tongs.  —  Several  differ- 
ent designs  of  scorifier  tongs  are 
employed,  the  first  and  oldest  being 
shown  in  Fig.  14.  The  curved  fork 
fits  the  bottom  of  the  scorifier 
while  the  long  arm  extends  across 
the  top.  These  are  preferably 
made  of  f  by  £  inch  steel  and 
should  be  about  30  inches  long. 
They  may  be  flattened  enough  at 
the  bend  to  give  the  right  degree 
of  spring.  Several  different  sizes 
should  be  supplied  to  handle  the 
different  sizes  of  scorifiers,  although 
a  pair  made  with  a  space  of  If 
inches  between  the  two  members  of  ^ 
the  fork  will  handle  2J,  2J  and  3  inch  scorifiers  perfectly.  The 
form  of  crucible  tongs  illustrated  in  Fig.  12  is  also  occasionally 
used  for  handling  scorifiers.  With  these,  scorifiers  may  be  lifted 
from  the  rear  of  the  muffle  without  disturbing  those  in  front.  They 
are  convenient  in  that  one  pair  of  tongs  will  fit  any  size  of  scorifier. 


» 


36  A   TEXTBOOK  OF  FIRE  ASSAYING 

Cupel  Tongs.  —  A  good  form  of  cupel  tongs  is  illustrated  in 
Fig.  15.  It  may  be  made  of  half-inch  half-round  stock  and  should 
be  about  30  inches  long.  It  is  best  to  curve  the  points  of  these 
tongs  to  conform  to  the  cupel,  so  that  if  the  operator  happens 
to  grasp  a  cupel  below  its  center  of  gravity  it  cannot  turn  over  and 
spill  the  contents.  For  handling  a  large  number  of  cupels  at  one 
time,  a  cupel  shovel  of  light  weight  iron  is  often  used.  Thig 
may  be  made  of  any  convenient  width  and  from  24  to  30  inches 
long.  The  cupels  are  moved  on  or  off  the  shovel  with  a  rake  or 
rabble  of  the  same  width. 


FIG.  15.— Cupel  tongs. 


FIG.  16. — Annealing  cup  tongs. 

Annealing  and  Parting  Cup  Tongs.  —  A  pair  of  tongs  arranged 
to  handle  annealing  cups  is  shown  in  Fig.  16.  They  should  be 
made  so  that  when  closed  they  fit  the  cup  somewhat  above  its 
center.  When  a  large  number  of  annealings  are  to  be  done  at 
one  time  the  cups  may  be  placed  in  some  form  of  clay  dish  and 
all  put  in  the  muffle  together. 


FIG.  17. — Muffle  scraper. 


FIG.  18. — Muffle  spatula. 

Muffle  Scrapers  and  Spatulas.  —  The  muffle  scraper,  as  its 
name  implies,  is  a  tool  intended  for  the  prompt  removal  of  any- 
thing spilled  upon  the  floor  of  the  muffle.  A  muffle  spatula  is  a 
long  rod  of  say  \  inch  iron,  flattened  at  the  end.  It  is  useful  in 
spreading  bone-ash  over  a  slagged  spot  in  the  muffle,  as  well  as 


FURNACES  AND  FURNACE  ROOM  SUPPLIES 


37 


in  adding  reagents,  etc.,  to  crucibles  and  scorifiers  already  in  the 

muffle.     In  Figs.  17  and  18  are  shown  a  muffle  scraper  and  spatula. 

Molds.  —  Various  forms  of  molds  to  receive  the  molten  charge 

are  in  use.     They  are  usually  made  of  cast  iron  and  should  be 


FIG.  19.  —  Four-hole  crucible  mold. 

machined  on  the  inner  surface.  For  crucible  fusions,  the  writer 
prefers  one  having  a  fairly  sharp  (50°)  conical  cavity  holding  about 
60  cubic  centimeters  and  with  a  slightly  rounded  bottom.  In  Fig. 


FIG.  20.  —  Cupel  tray  holding  16  cupels. 

19  is  shown  a  four-hole  mold  of  this  description.  This  leaves  the 
lead  button  in  good  shape  for  pounding  and  permits  a  good  separa- 
tion of  lead  and  slag.  Some  assayers  prefer  a  solid  block  mold 


38  A   TEXTBOOK  OF  FIRE  ASSAYING 

with  a  conical  cavity,  claiming  that  the  fusions  cool  more  rapidly. 
A  mold  of  this  type,  however,  is  heavier  to  handle  and  more  expen- 
sive. If  a  muffle  full  of  crucibles  is  poured  at  one  time,  it  will  be 
found  that  those  first  poured  are  ready  for  slagging  almost  imme- 
diately, even  if  the  lighter  molds  are  used. 


FIG.  21.  —  Clay  dish  holding  24  annealing  cups. 

For  scorification  fusions,  molds  with  smaller  cavities  are  used. 
They  are  made  with  spherical  or  with  flatly-coned  cavities  and 
both  types  are  satisfactory.  A  convenient  form  of  mold  is  one 
in  which  the  number  of  cavities  equals  the  number  of  scorifiers 
which  the  muffle  will  hold. 

Cupel  Trays,  etc.  —  A  convenient  cupel  tray  is  illustrated  in 
Fig.  20.  A  clay  dish  for  annealing  is  shown  in  Fig.  21.  Any 
form  of  hammer  will  serve  for  slagging  the  buttons,  but  one  with 
a  round  section  is  preferable.  Ten-inch  forceps  are  satisfactory 
both  for  holding  buttons  while  slagging  and  for  removing  the 
nails  from  iron-nail  fusions. 


CHAPTER  III. 
ORE  SAMPLING. 

A  sample  is  a  small  amount  which  contains  all  the  components 
in  the  proportions  in  which  they  occur  in  the  original  lot. 

Object.  —  The  object  of  sampling  an  ore  is  to  obtain,  for 
chemical  or  mechanical  tests,  a  small  amount  which  shall  con- 
tain all  the  minerals  in  the  same  proportion  as  the  original  lot. 
In  the  subsequent  discussion  the  word  "  sample  "  will  be  taken  to 
mean  that  fraction  which  is  taken  to  represent  the  whole,  whether 
or  not  it  does  so.  The  compound  words  correct-sample, 
representative-sample,  true-sample,  will  be  used  to  represent  the 
ideal  conditions. 

In  the  intelligent  operation  of  a  mine  or  metallurgical  plant, 
it  is  necessary  to  sample  and  assay  continually.  In  most  mines, 
the  different  faces  of  ore  are  sampled  every  day.  In  concen- 
trating plants,  it  is  customary  to  sample  the  products  of  every 
machine  at  frequent  and  regular  .intervals,  to  ascertain  whether 
the  machine  is  doing  the  work  expected  of  it.  In  smelters,  it  is 
necessary  to  sample  and  assay  every  lot  of  ore,  as  well  as  fluxes 
and  fuels,  in  order  to  calculate  a  charge  which  will  run  properly 
in  the  furnace.  The  slag,  flue  dus«t  and  metallic  products  must 
also  be  sampled  and  assayed,  with  a  view  to  maintaining  control 
of  the  operations.  In  lixiviation  plants,  the  ore  and  tailings, 
as  well  as  the  solutions,  must  be  sampled  in  order  that  the  daily 
work  of  the  plant  may  be  controlled  and  checked.  In  fact, 
careful  sampling  and  assaying  cannot  be  disregarded,  and  are 
becoming  more  and  more  important  every  day  as  the  grade  of  ore 
decreases  and  the  margin  of  profit  becomes  less. 

The  assayer  will  usually  have  the  major  part  of  the  sampling 
done  for  him,  but  he  is  expected  to  know  how  to  do  it  when  called 
upon.  He  usually  has  to  prepare  only  the  final  sample,  but  will 
occasionally  receive  lots  of  10  to  100  or  more  pounds  to  assay,  in 
which  case  he  will  have  to  do  his  own  sampling.  The  following 
discussion  will  deal  principally  with  the  assay  laboratory  problems 

39 


40  A   TEXTBOOK  OF  FIRE  ASSAYING 

of  sampling;  the  question  of  mine  sampling  is  entirely  omitted, 
but  methods  used  in  sampling  mills  are  briefly  reviewed  for  the 
sake  of  completeness. 

Methods.  —  The  question  of  ore  sampling  is  probably  the 
most  complicated  of  all  sampling  problems,  because  of  the  great 
variety  of  constituents  and  the  lack  of  uniformity  in  their  distri- 
bution throughout  the  whole  mass.  It  is  obvious  that,  however 
we  may  proceed,  the  problem  is  to  select  a  method,  such  that 
every  particle  of  our  non-homogeneous  mixture,  the  ore,  shall 
have  nearly  the  same  chance  of  being  included  in  the  sample. 
Several  methods  may  be  followed  to  secure  this  result  and.  assum- 
ing the  ore  to  have  had  a  preliminary  crushing,  the  available 
methods  are: 

1.  Random  selection 

2.  Selection  by  rule 

3.  Mixing  and  cutting 

The  first  two  are  rough,  preliminary  methods  generally  known 
as  "  grab-sampling."  The  last  is  capable  of  mathematical  pre- 
cision and  may  be  repeated  through  all  stages  of  the  sampling 
process.  It  is  the  only  method  which  should  be  used  when  an 
exact  sample  of  the  precious  metal  ores  is  desired.  Iron  ores  are 
so  uniform  that  "  grab-sampling  "  is  likely  to  yield  satisfactory 
results. 

When  it  is  considered  that  the  final  sample  for  chemical  analy- 
sis usually  weighs  only  half  a  gram  and  for  fire  assay  somewhat 
less  than  15  grams,  and  that  each  must  truly  represent  from  1  to  5 
carloads  of  ore  weighing  from  50  to  250  tons,  the  enormous  prac- 
tical difficulties  of  the  problem  may  be  appreciated. 

Precise  sampling  may  usually  be  considered  to  consist  of  three 
distinct  operations,  repeated  as  many  times  as  necessary.  These 
operations  are  crushing,  mixing  and  cutting.  The  cutting  gives 
a  sample  and  a  reject.  By  a  repetition  of  the  three  operations  the 
sample  may  be  further  reduced  until  it  has  reached  the  desired 
weight. 

The  whole  science  of  ore  sampling  depends  primarily  on  a 
correct  knowledge  of  the  proper  relation  between  the  maximum 
size  of  the  ore  particles  and  the  weight  of  the  sample  taken.  The 
problem  to  be  solved  in  each  case  is  somewhat  as  follows:  when 
a  particular  ore  has  been  crushed  to  a  certain  size,  how  small  a 
sample  is  it  safe  to  take  from  this  and  still  keep  within  the  limit 


ORE  SAMPLING  41 

of  allowable  error?  It  is  necessary  to  know  the  ore,  the  limit  of 
allowable  error,  and  the  mathematical  principles  involved. 

Sampling  is  classed  as  hand  sampling  when  the  mixing  and  cut- 
ting down  is  done  by  men  with  shovels,  and  as  machine  sampling 
when  it  is  done  by  some  form  of  automatic  machine. 

Commercial  Considerations.  —  The  most  certain  method  of 
obtaining  a  representative  sample  of  a  lot  of  ore  would  be  to  crush 
the  whole  to  100-,  120-mesh  or  finer,  mix  it  thoroughly  and  then 
cut  down  to  the  desired  weight.  This  method  can  be  followed  for 
small  amounts  of  a  pound  or  so,  but  in  the  case  of  large  lots  it 
would  entail  too  much  labor  and  would  usually  unfit  the  ore  for 
future  treatment.  The  method  generally  adopted  is  a  compro- 
mise and  consists  in  crushing  the  whole  lot  to  a  certain  predeter- 
mined maximum  size  and  then  taking  out  a  certain  fraction  as  a 
sample.  This  sample  is  again  crushed  to  a  smaller  size  and  cut 
down  as  before,  and  this  process  repeated  until  finally  the  assay 
sample  is  obtained. 

The  care  which  is  required  in  sampling,  as  well  as  the  size  to 
which  a  lot  of  ore  or  other  material  must  be  crushed  before  a 
sample  is  taken,  depends  upon  the  value  and  uniformity  of  com- 
position of  the  material.  The  more  uniform  it  is,  the  smaller 
may  be  the  sample  taken  after  crushing  to  any  particular  size. 
For  instance,  in  the  case  of  a  solid  piece  of  galena  containing  sil- 
ver uniformly  distributed  as  an  isomorphous  silver  sulphide,  a 
piece  may  be  broken  off  anywhere,  and  after  being  crushed,  will 
give  a  lot  of  ore  which  is  truly  a  sample  of  the  piece.  If,  however, 
the  specimen  is  not  solid  galena,  but  is  made  up  of  galena  and 
limestone,  the  silver  still  being  contained  in  the  galena,  it  will 
be  necessary  to  crush  the  whole  lot  to  a  uniformly  fine  size  be- 
fore taking  out  a  fractional  part  for  a  sample.  Furthermore, 
it  will  readily  be  seen  that  the  greater  the  difference  in  the  grade 
of  the  different  minerals  in  the  ore,  the  finer  the  ore  must  be 
crushed  before  a  sample  of  a  given  size  should  be  taken  from  it. 

Since  ores  are  never  perfectly  uniform  in  composition,  a  cer- 
tain amount  of  crushing  is  evidently  necessary  in  every  case. 
To  determine  the  amount  of  crushing  it  is  important  to  consider 
the  commercial  side  of  the  question,  that  is,  to  determine  how 
far  it  will  pay  to  go  with  the  process.  Evidently  a  mistake  of 
1  per  cent  in  the  iron  contents  of  a  carload  of  iron  ore  worth 
$3  a  ton  is  less  serious  than  the  same  percentage  error  in  the 


42  A   TEXTBOOK  OF  FIRE  ASSAYING 

copper  contents  of  a  car  of  copper  ore  worth  $50  a  ton.  There- 
fore it  may  be  seen,  that  it  will  pay  the  seller  or  buyer  of  the 
copper  ore  to  go  to  more  pains  and  expense  in  the  sampling  of 
the  ore,  than  if  he  were  dealing  with  the  less  valuable  iron  ore. 
The  commercial  conditions  are  met  when  the  ultimate  sample 
obtained  comes  within  an  allowable  limit  of  error,  usually  1  per 
cent,  of  the  ideal  or  true  figure,  provided  also  that  it  has  been 
obtained  without  undue  delay  and  at  a  reasonable  cost. 

PRINCIPLES  OF  SAMPLING. 

Varying  Relation  of  Size  of  Sample  to  Maximum  Particle.  — 
Every  ore-sampling  operation  is  in  effect  a  laboratory  experiment 
in  probability,  and  the  variation  of  any  portion  or  sample  of  a  lot 
from  the  average  composition  of  the  whole  may  be  considered 
to  be  due  to  the  excess  or  deficit  of  one  or  more  particles  of  the 
ore. 

The  effect  upon  the  results  will  be  greatest  when  the  piece  or 
pieces  which  are  in  excess  or  deficit  are  of  the  largest  size,  great- 
est specific  gravity  and  greatest  variation  in  quality  from  the 
average. 

Disregarding  for  the  moment  the  last  two  of  these  factors  and 
supposing  the  ore  particles  to  be  approximately  uniform  in  size, 
it  is  evident  that  the  sample  must  contain  so  many  particles  that 
one  additional  particle  of  the  richest  mineral  would  cause  prac- 
tically no  variation  in  the  value.  This  means  that  the  sample  of 
the  ordinary  ore  must  contain  a  very  large  number  of  particles 
500,000  in  some  cases,  5,000,000  in  others. 

Having  determined  how  many  particles  of  the  ore  it  is  necessary 
to  include  in  the  sample,  and  assuming  the  different  minerals  to 
be  entirely  detached  from  one  another,  it  would  be  fair  to  take 
such  a  weight  of  ore  after  each  reduction  as  would  contain  this 
established  number  of  particles.  Or,  as  the  weight  of  a  lump  is 
proportional  to  the  cube  of  its  diameter,  the  weight  of  ore  taken 
for  the  sample  should  be  proportional  to  the  cube  of  the  diameter 
of  the  largest  particle  of  the  ore. 

In  the  ordinary  ore,  however,  the  different  minerals  are  not 
entirely  detached  from  one  another,  but  approach  this  condition 
more  and  more  closely  as  the  size  of  the  ore  is  reduced.  Hence  a 
fixed  number  of  the  particles  of  the  fine  ore  is  less  likely  to  be  a 
true  average  of  the  whole  than  the  same  number  of  pieces  of  the 


ORE  SAMPLING 


43 


lump  ore  before  it  was  broken.  Therefore  as  the  size  of  the  ore 
is  reduced  a  larger  and  larger  number  of  particles  should  be  taken 
for  the  sample.  To  conform  to  this  condition  the  following  rule 
was  proposed  by  Professor  R.  H.  Richards:  "  For  any  given  ore 
the  weight  taken  for  a  sample  should  be  proportional  to  the  square 
of  the  diameter  of  the  largest  particle.  " 

The  accompanying  table,  based  on  figures  taken  from  the  prac- 
tice of  several  careful  managers,  to  a  certain  extent  conforms  to 
this  rule.  The  table  was  arranged  and  is  now  published  with 
the  permission  of  Professor  Richards. 

TABLE  V. 
WEIGHTS  TO  BE  TAKEN  IN  SAMPLING  ORE. 


l 

2 

3 

4 

5 

6 

Weights  of 

Diameter  of  largest  particles  —  millimeters 

sarnpl© 

pounds 

Very  low 

Low 

Very 

grade  or 
very   uni- 

grade or 
uniform 

Medium  ores 

spotted 

rich   and' 
spotted 

form  ores 

ores 

ores 

ores 

20,000.000 

207.00 

114.00 

76.20 

50.80 

31.60 

5.40 

10,000.000 

147.00 

80.30 

53.90 

35.90 

22.40 

3.80 

5,000.000 

107.00 

56.80 

38.10 

25.40 

15.80 

2.70 

2,000.000 

65.60 

35.90 

24.10 

16.10 

10.00 

1.70 

1,000.000 

46.40 

25.40 

17.00 

11.40 

7.10 

1.20 

500.000 

32.80 

18.00 

12.00 

8.00 

5.00 

.85 

200.000 

20.70 

11.40 

7.60 

5.10 

3.20 

.54 

100.000 

14.70 

8.00 

5.40 

3.60 

2.20 

.38 

50.000 

10.70 

5.70 

3.80 

2.50 

1.60 

.27 

20.000 

6.60 

3.60 

2.40 

1.60 

1.00 

.17 

10.000 

4.60 

2.50 

1.70 

1.10 

.71 

.12 

5.000 

3.30 

1.80 

1.20 

.80 

.50 

2.000 

2.10 

1.10 

.76 

.51 

.32 

1.000 

1.50 

.80 

.54 

.36 

.22 

.500 

1.00 

.57 

.38 

.25 

.16 

.200 

.66 

.36 

.24 

.16 

.10 

.100 

.46 

.25 

.17 

.11 

.050 

.33 

.18 

.12 

.020 

.21 

.11 

.010 

.15 

.005 

.10 

The  first  column  shows  the  safe  weight  in  pounds  for  a  sample 
of  ore  of  any  of  the  six  grades  shown  and  for  sizes  as  indicated  in 
the  respective  columns.  Column  1  applies  to  iron  ores,  column  2 


44  A   TEXTBOOK  OF  FIRE  ASSAYING 

to  low-grade  lead,  zinc  and  copper  ores  and  even  to  low-grade 
pyritic  gold  ores,  without  native  gold,  where  the  pyrite  is  evenly 
distributed  through  the  ore.  Columns  3  and  4  apply  to  ores  in 
which  the  valuable  minerals  are  less  uniformly  distributed. 
Columns  5  and  6  apply  to  ore  containing  fine  particles  of  native 
gold  or  silver,  also  to  telluride  and  other  "  spotty  ores.  " 

It  should  be  remembered  that  the  above-mentioned  rules  for 
sampling  will  not  hold  for  ore  containing  large  pieces  of  malleable 
minerals  such  as  native  gold,  silver,  silver  sulphide,  chloride  etc. 
These  roll  out  and  do  not  crush  and  must  be  treated  by  special 
methods.  See  "  Sampling  Ores  Containing  Malleable  Minerals." 

In  using  the  table,  it  is  not  necessary  to  crush  successively  to 
all  of  the  sizes  shown  in  any  of  the  columns.  The  ore  may  be 
crushed  to  any  fineness  convenient  and  then  a  sample  of  the  weight 
shown  in  the  table  may  be  taken.  In  sampling-mill  practice  it  is 
customary  to  reduce  the  diameter  of  the  coarsest  particles  one- 
half  at  each  stage  or  crushing,  thus  reducing  the  volume  to  one- 
eighth  or  12.5  per  cent.  It  is  also  customary  in  practice  to  take 
a  20  per  cent  sample  at  each  stage;  consequently  the  ratio  be- 
tween the  weight  of  sample  and  size  of  maximum  particle  is  con- 
stantly increasing  throughout  the  sampling  process,  thereby  meet- 
ing theoretical  conditions  previously  discussed. 

Relation  of  Size  of  Sample  to  Grade  of  Ore  and  Effect  of 
Specific  Gravity  of  Richest  Mineral.  —  Although  it  had  long  been 
appreciated  that  the  size  of  the  sample  would  have  to  be  greater 
as  the  ratio  of  the  grade  of  the  richest  mineral  to  the  average 
grade  increased,  it  remained  for  Brunton*  to  develop  a  formula 
by  which  the  proper  ratio  between  these  could  be  scientifically 
controlled.  According  to  him,  each  of  the  following  factors  must 
be  included  in  any  formula  to  be  used  for  the  control  of  sampling- 
operations. 

W  =  weight  of  sample  in  pounds. 
k  =  grade  of  richest  mineral  in  ounces  per  ton. 
c  =  average  grade  of  ore  in  ounces  per  ton. 
s  =  specific  gravity  of  richest  mineral. 
n  =  number  of  maximum-sized  particles  of  richest  mineral  in 

excess  or  deficit  in  sample. 
/  =  a  factor  expressing  the  ratio  of  the  actual  weight  of  the 

*  Trans.  A.I.M.E.  25,  p.  826  (1895). 


ORE  SAMPLING  45 

largest  particle  of  richest  mineral  which  will  pass  a  screen 

of  a  given  size  to  the  weight  of  the  largest  cube  of  the 

same  mineral  which  will  pass  the  screen. 
p  =  allowable  percentage  error  in  sample. 
D  =  diameter  in  inches  of  the  holes  in  the  screen,  or  other 

normal  diameter  to  which  the  ore  is  crushed. 

From  these  Brunton  finds 

3   

Wcp 


D  =  -Q5Vfsn(k-c} 

Making  p,  the  allowable  percentage  error,  =  1,  the  formula 
becomes 


i 


D  =  .65  t/       fc 

fsn(k  —  c) 

To  determine  a  value  to  use  for  n,  he  made  a  number  of  assays 
on  two  different  lots  of  high-grade  ore  crushed  to  pass  a  certain 
limiting  screen.  The  average  deviation  from  the  mean  =  p  was 
substituted  in  the  formula,  and  results  of  2.64  and  3.14  respec- 
tively were  found  for  n.  Assuming  that  3  is  a  safe  value  for  n 
and  cubing  each  side  we  find 

£3.  _J^ , 

W.Sfs(k  -  c) 
or 

TT7       10.8/s£>3(/c  -  c) 


from  which  may  be  found  the  safe  weight  in  pounds  for  a  sample 
of  any  ore  whose  largest  particle  is  D  inches.  Taking  four  ex- 
amples, using  as  the  richest  minerals  pyrite,  galena,  native  silver 
and  native  gold  and  assuming  different  values  for  D,  k,  c  and  /  the 
following  table  was  made  after  the  style  of  the  table  first  shown 
in  Hof man's  "  Metallurgy  of  Lead."  The  values  for  /  used  for 
the  fine  sizes  were  those  determined  by  Brunton's  experiments, 
i.e.,  4  for  pyrite  and  galena  and  6  for  native  silver  and  native  gold. 
This  value  of  /  is  reduced  gradually,  until  for  1  inch  diameter,  it 
is  made  equal  to  1,  this  variation  therefore  tending  to  compensate 
for  the  greater  uniformity  of  value  of  the  particles  as  they  become 
larger. 


46 


A   TEXTBOOK  OF  FIRE  ASSAYING 


The  following  table  is  probably  the  best  and  certainly  the  most 
conservative  of  all.  A  good  deal  of  intelligent  discrimination  may 
often  be  used,  however,  and  mere  formulas  can  never  be  made  to 
cover  all  possible  contingencies.  For  instance,  in  sampling  an  ore 
in  which  the  valuable  mineral  is  finely  and  uniformly  disseminated 

TABLE  VI. 
WEIGHTS  TO  BE  TAKEN  IN  SAMPLING  ORE. 


Size  of 

Safe  weight  in  pounds  when  largest  particles  are  of  size  given  in 

&  § 

particles 

second  column 

61 

bo   g 

I 

11 

Grade  of  richest  mineral  divided  by  average  grade 

Q  ^ 

10 

50 

200 

600 

1,500 

2,500 

120 

.0043 

.003 

.010 

.025 

.043 

100 

.0055 

.0003 

.0018 

.007 

.021 

.053 

.089 

50 

.0100 

.0017 

.0095 

.039 

.116 

.291 

.485 

5.0 

14 

.0364 

.0585 

.319 

1.29 

3.90 

9.76 

16.3 

4 

.145 

2.96 

16.1 

65.5 

195. 

494. 

823. 

2 

.338 

30.0 

163. 

664. 

2,000. 

5,000. 

8,340. 

.5 

75.9 

413. 

1,680. 

5,050 

12,600 

21,100. 

1.0 

486. 

2,650. 

10,700. 

32,300. 

80,900. 

140,000. 

120 

.0043 

.005 

.015 

.038 

.064 

100 

.0055 

.0005 

.0027 

.011 

.032 

.080 

.134 

50 

.0100 

.0026 

.0143 

.058 

.174 

.437 

.727 

7.5 

14 

.0364 

.0878 

.479 

1.94 

5.85 

14.6 

24.5 

4 

.145 

4.44 

24.2 

98.3 

293. 

740. 

1,230. 

2 

.338 

45.0 

245. 

996. 

3,000. 

7,500. 

12,500. 

.5 

114. 

620. 

2,520. 

7,580. 

19,000. 

31,600. 

1.0 

729. 

3,970. 

16,100. 

48,500. 

121,000. 

211,000. 

120 

.0043 

.0005 

.0027 

.011 

.032 

.081 

.135 

100 

.0055 

.0010 

.0055 

.022 

.068 

.170 

.283 

50 

.0100 

.0041 

.0222 

.090 

.272 

.679 

1.13 

10.5 

14 

.0364 

.148 

.804 

3.26 

9.83 

24.6 

41.0 

4 

.145 

7.78 

42.4 

172. 

518. 

1,300. 

2,160. 

2 

.338 

78.8 

429. 

1,740. 

5,250. 

13,100. 

21,900. 

.5 

230. 

1,250. 

5,080. 

15,300 

38,200. 

63,800. 

1,500 

3,000 

6,000 

15,000 

30,000 

00,000 

150 

.0036 

.0798 

.159 

.319 

.798 

1.59 

3.19 

120 

.0043 

.136 

.272 

.544 

1.36 

2.72 

5.40 

100 

.0055 

.284 

.569 

1.14 

2.84 

5.69 

11.4 

17.6 

50 

.0100 

1.14 

2.28 

4.56 

11.4 

22.8 

45.6 

14 

.0364 

41.2 

82.5 

165. 

412. 

825. 

1,650. 

4 

.145 

2,170. 

4,350. 

8,690. 

21,700. 

43,500. 

86,900. 

2 

.338 

22,000. 

44,000 

88,100. 

220,000. 

440,000. 

881,000. 

ORE  SAMPLING  47 

throughout  the  gangue,  a  much  smaller  sample  than  that  given  in 
the  table  may  be  taken  for  the  coarse  sizes,  although  for  the  fine 
sizes  the  full  quantities  shown  in  the  table  should  be  taken.  An- 
other ore,  with  perhaps  the  same  ratio  of  value  of  the  richest 
mineral  to  average  grade,  having  the  rich  mineral  in  larger  crystals 
or  masses,  will  have  to  be  sampled  as  carefully  as  indicated  by 
the  table  throughout  the  entire  operation. 

It  should  be  noted  also,  that  except  in  the  case  of  native  metals, 
the  richest  minerals  are  usually  more  finely  divided  by  crushing 
than  the  gangue;  therefore  the  extreme  case  provided  for  by  the 
formula  is  seldom  met  in  practice. 

One  of  the  most  difficult  things  an  assayer  may  be  called  upon 
to  do  is  to  sample  such  mill  products  as  vanner  concentrates. 
In  these  the  particles  of  gangue  minerals  are  two  or  three  times 
the  diameter  of  the  average  rich  mineral  and  good  mixing  is 
impossible.  The  material  stratifies  whenever  handled  and  the 
greatest  care  must  be  taken  if  the  sampling  is  to  be  successful. 

SAMPLING   PRACTICE. 

Recording.  —  Every  lot  of  ore  coming  into  an  assay  office, 
laboratory,  custom  mill  or  smelter  should  be  given  a  lot  number 
which  should  never  be  repeated.  The  lot  should  be  immediately 
labeled  with  this  number.  A  record  book,  kept  for  this  purpose, 
should  show  the  number  of  the  sample,  date  of  receipt,  name  of 
mine,  company  or  individual  from  whom  received,  the  gross  and 
net  weight,  as  well  as  notes  on  the  general  mineral  character,  etc. 

Weighing.  —  Large  lots  of  ore  are  first  weighed,  and  a  moisture 
sample  is  sometimes  taken  at  this  point.  Small  lots  may  be  first 
dried  and  then  weighed. 

Crushing.  —  All  of  the  ore,  unless  already  fine  enough,  is  broken 
or  crushed  to  pass  a  screen  of  some  limiting  size.  This  size  de- 
pends upon  the  value  of  the  ore  and  other  factors  to  be  considered 
later.  The  finer  the  pulp  is  crushed,  the  more  uniform  in  size  are 
the  particles  and  more  thorough  mixing  and  better  sampling  is 
possible.  If  the  ore  is  to  be  smelted,  most  of  it  should  be  left  in 
the  coarse  state,  as  fine  ore  is  undesirable.  If  it  is  to  be  roasted  or 
leached,  on  the  other  hand,  fine  ore  is  not  objectionable,  and  the 
first  crushing  may  be  carried  further.  As  a  rule,  however,  the  aim 
is  to  minimize  the  crushing,  thus  saving  in  cost  and  keeping  down 
the  dust. 


48  A    TEXTBOOK  OF  FIRE  ASSAYING 

Machines  for  crushing  should  be  rapid  in  action  and  easy  to 
clean.  Jaw  breakers  and  rolls  fulfill  these  requirements;  ball 
mills  and  pebble  mills  do  not. 

Mixing.  —  This  step  in  the  process  of  sampling  is  often  omitted 
or  allowed  to  take  care  of  itself.  It  is  a  necessary  forerunner  of 
quartering  and  channeling,  but  is  usually  omitted  before  the  other 
methods  of  cutting.  Especially  in  the  handling  of  small  lots  of 
ore  in  the  laboratory,  it  is  best  to  be  over-careful  in  this  particular 
rather  than  the  reverse,  and,  as  it  adds  but  little  labor,  to  give  each 
lot  of  crushed  ore  a  thorough  mixing  before  cutting.  The  mixing 
of  small  lots  will  be  discussed  under  the  head  of  finishing  the 
sample. 

The  final  step  in  the  sequence  of  sampling  operations  consists  in 
taking  out  a  fraction  of  the  whole,  say  a  quarter  or  a  half,  in  some 
systematic,  impartial  manner.  The  part  taken  out  is  called  the 
sample,  and  the  operation  of  taking  it  is  the  cutting. 

Hand  Cutting.  —  The  following  methods  of  hand  cutting  are 
occasionally  used,  but  whenever  possible  are  being  replaced  by 
machine  cutting. 

FRACTIONAL  SHOVELING.  —  This  is  a  rough  starting  method, 
suited  only  to  large  lots  of  low-grade  or  fairly  uniform  ore.  When 
the  ore  is  being  taken  away  from  the  crusher  or  shoveled  out  of 
cars,  as  the  case  may  be,  every  second,  third,  fifth,  or  tenth  shovel- 
ful, depending  on  the  value  and  uniformity  of  the  ore,  is  taken 
and  placed  in  a  separate  pile,  which  is  afterwards  cut  down  by 
some  of  the  methods  described  later.  When  the  ore  is  being 
shoveled,  care  must  be  taken  that  each  shovelful  is  taken  from  the 
floor.  Lumps  which  are  too  large  for  the  shovel  should  be  broken 
and  put  back  on  the  pile.  The  method  is  open  to  the  serious  ob- 
jection that  it  is  a  very  simple  matter  for  a  prejudiced  person 
to  make  the  sample  either  higher  or  lower  in  grade  than  the  av- 
erage, by  selection  of  his  shovel  samples. 

QUARTERING.  —  This  is  the  method  of  cutting  which  accom- 
panies coning.  It  presupposes  a  thorough  mixing  by  coning,  as 
the  two  always  go  together. 

CONING.  —  The  sample  is  shoveled  into  a  conical  pile,  each 
shovelful  being  thrown  upon  the  apex  of  the  cone  so  that  it  will 
run  down  evenly  all  around.  When  a  large  lot  of  ore  is  to  be 
mixed  by  coning,  it  is  first  dumped  in  a  circle  and  then  coned  by 
one  or  more  men  who  walk  slowly  around  between  the  cone  and 


ORE  SAMPLING  49 

the  circle  of  ore.  The  best  results  are  obtained  by  coning  around 
a  rod,  as  by  this  means  the  center  of  the  cone  is  kept  in  a  vertical 
line.  Coning  does  not  thoroughly  mix  an  ore,  but  rather  sorts  it 
into  fine  material  which  lies  near  the  center  and  coarser  material 
which  rolls  down  the  sides  of  the  cone.  If  the  ore  is  practically 
uniform  in  size  and  specific  gravity,  the  mixing  may  be  more  thor- 
ough. A  slight  dampening  of  the  ore  is  said  to  allow  of  better 
mixing  by  coning.  The  floor,  for  this  and  other  hand  sampling 
operations,  should  be  smooth,  and  free  from  cracks  which  would 
make  good  cleaning  difficult  or  impossible.  A  floor  made  of  sheet- 
iron  or  steel  plates  is  preferable. 


FIG.  22.  —  Cone  of  crushed  ore. 

Figure  22,*  a  cone  of  crushed  ore,  shows  clearly  the  inherent 
defect  of  this  method  of  sampling,  the  segregation  of  coarse  and 
fine  ore,  caused  by  dropping  shovelful  after  shovelful  on  top  of  a 
cone. 

When  the  cone  is  completed,  it  is  worked  down  into  the  form  of 
a  flat  truncated  cone  by  men  who  walk  around  and  around,  draw- 
ing their  shovels  from  center  to  periphery,  or  starting  at  the  apex 
and  working  the  shovel  up  and  down  in  the  path  of  a  spiral. 
The  point  to  be  observed  here  is  not  to  disturb  the  radial  dis- 
tribution of  the  coarse  and  fine  ore.  After  flattening,  the  cone 
is  divided  into  four  90-degree  sectors  or  quarters  by  means  of  a 
sharp-edged  board,  or  better,  by  a  steel-bladed  quarterer.  These 

*  From  U.  S.  Bureau  of  Mines  Technical  Paper  No.  86:  Ore  Sampling 
Conditions  in  the  West. 


50 


A   TEXTBOOK  OF  FIRE  ASSAYING 


quarters  should,  of  course,  radiate  from  the  position  of  the  center 
of  the  original  cone.  Two  opposite  quarters  are  taken  out  and 
rejected  and  the  two  others  are  then  taken  for  the  sample.  Care 
must  be  taken  at  this  point  to  sweep  up  all  dust  belonging  to  the 


FIG.  23.  —  Partly  flattened  cone 


FIG.  24.  —  Truncated  cone  from  which  reject  quarters  have  been  removed. 

rejected  portions  before  proceeding,  so  that  this  dust  shall  neither 
be  lost  nor  mixed  with  the  sample.  This  sample  may  be  again 
mixed  by  coning  and  quartered,  or  crushed,  coned  and  quartered 
as  the  case  may  require. 


ORE  SAMPLING  51 

Figure  23*  shows  a  partly  flattened  cone  and  Fig.  24*  a  cake 
from  which  the  reject  quarters  have  been  removed. 

When  properly  carried  out,  this  method  may  be  made  to  yield 
fairly  accurate  results,  but  at  best  it  is  a  slow  and  tedious  process, 
and  requires  the  most  conscientious  work  on  the  part  of  the  la- 
borers to  ensure  correct  results.  It  is  open  to  the  objection  that 
it  affords  opportunity  for  manipulation  of  the  sample  by  dishonest 
operators. 

Coning  and  quartering  is  the  old  Cornish  method  of  ore  sampling 
and  was  almost  universally  used  thirty  years  ago.  It  is  still  used 
to  some  extent  as  a  finishing  method  at  sampling  works  and  by 
engineers  in  the  field  where  no  machinery  is  available. 

BENCH  SYSTEM  OF  CONING.  —  The  tendency  to  segregate, 
which  is  the  principal  objection  to  coning,  can  be  largely  overcome 
by  what  is  known  as  the  bench  system  of  coning.  Under  this 
system  all  of  the  ore  is  not  piled  in  a  single  cone;  a  part  of  it  is 
coned  first  and  this  small  cone  is  worked  out  into  a  layer  of  con- 
siderable diameter  and  but  little  thickness.  Another  part  of  the 
ore  is  then  coned  on  top  of  this  and  the  cone  truncated.  This  is 
repeated  until  all  of  the  ore  is  used.  This  method  is  said  to  give 
working  results  which  are  much  more  satisfactory  than  those 
obtained  by  the  regular  system. 

RIFFLE  CUTTING.  —  Riffle  cutting  or  splitting  is  the  most  ac- 
curate laboratory  method  available.  The  riffle,  splitter  or  split- 
shovel  consists  of  a  number  of  parallel  troughs  with  open  spaces 
between  them,  the  spaces  usually  being  of  the  same  width  as  the 
troughs.  These  troughs  are  rigidly  fastened  together  and  either 
provided  with  a  handle,  making  a  split-shovel,  or  set  up  at  an 
angle  of  about  45°  making  an  inclined  riffle.  Figure  25  shows  a 
split-shovel  with  pan  and  shovel.  These  may  be  made  in  different 
sizes  but  are  useful  only  for  small-scale  work. 

The  ore  is  taken  up  on  a  flat  shovel  or  special  pan  and  spread 
over  the  troughs,  care  being  taken  not  to  heap  the  ore  above  the 
troughs.  Either  the  ore  which  falls  in  the  troughs  or  that  which 
falls  between  them  may  be  taken  as  the  sample.  The  cutting 
may  be  repeated  as  many  times  as  is  deemed  desirable.  For  the 
best  results  in  cutting  any  sample  of  ore  by  this  method,  care 
should  be  taken  to  have  only  a  thin  stream  of  ore  falling  from  the 

*  From  U.  S.  Bureau  of  Mines  Technical  Paper  No.  86:  Ore  Sampling 
Conditions  in  the  West. 


52  A   TEXTBOOK  OF  FIRE  ASSAYING 


FIG.  25.  —  Split  shovel  and  pans. 


FIG.  26.  —  Brunton  splitter. 


ORE  SAMPLING 


53 


pouring  pan  and  to  move  this  pouring  pan  back  and  forth  over 
the  split  shovel,  in  a  horizontal  direction  perpendicular  to  the 
riffles,  so  that  every  part  of  the  stream  of  ore  is  being  directed 
alternately  and  rapidly  first  into  the  sample  and  then  into  the 
reject.  The  more  irregular  in  size,  specific  gravity  and  value  are 
the  minerals,  the  greater  the  care  which  should  be  taken  in  this 
particular.  The  sample  should  be  mixed  before  recutting. 


FIG.  27.  —  Closed  type  splitter. 

A  modification  of  the  riffle  or  split-shovel  known  as  the  Jones 
sampler,  or  simply  as  a  "  splitter,"  is  in  general  the  most 
convenient  form  of  sampler  for  finishing  work.  It  is  a  riffle 
sampler  in  which  the  bottoms  of  the  riffles  are  steeply  in- 
clined, first  in  one  direction  and  then  in  the  other.  The  ore  is 


54  A   TEXTBOOK  OF  FIRE  ASSAYING 

spread  over  the  riffles  in  the  Jones  sampler  exactly  as  over  the 
split-shovel,  and  is  caught  in  two  pans  placed  underneath. 

The  Jones  splitter  and  those  similar  to  it  have  one  decided  ad- 
vantage over  the  flat  type  shown  in  Fig.  25,  in  that  the  riffles 
cannot  be  overloaded,  a  very  common  fault  of  the  shovel  type. 
In  Fig.  26  is  shown  a  very  substantial  form  known  as  the  Brunton 
riffle,  the  operation  of  which  is  self-evident. 

One  objection  to  the  Jones  sampler  and  other  similar  models, 
is  the  possibility  of  the  loss  of  considerable  fine  ore  dust,  due  to 
the  greater  length  of  fall  of  the  ore  before  coming  to  rest.  One 
way  to  obviate  this  would  be  to  slightly  moisten  the  thoroughly 
mixed  ore  before  cutting.  A  better  way  is  to  close  the  bottom 
of  the  sampler  and  set  it  directly  on  the  pans.  An  example  of 
this  type  of  splitter  is  shown  in  Fig.  27. 

In  selecting  a  split-shovel  or  riffle  cutter  for  any  particular  sam- 
pling operation,  care  should  be  taken  that  the  distance  between 
the  riffles  be  at  least  four  times  the  diameter  of  the  maximum  par- 
ticle of  ore.  It  is  found  that  a  slight  bridging  action  may  occur 
if  this  precaution  is  not  observed.  Riffle  cutting  is  the  most 
rapid  method  of  hand  sampling  and  is  also  the  most  accurate. 

Machine  Cutting.  —  A  large  number  of  machines  have  been 
devised  to  take  the  place  of  the  slow,  laborious  methods  of  hand 
sampling.  All  these  machines  depend  on  taking  the  sample  from 
a  stream  of  falling  ore.  They  may  be  classified  as  continuous 
and  intermittent  samplers.  The  continuous  samplers  take  part 
of  the  stream  all  the  time,  by  placing  a  partition  in  the  falling 
stream  of  ore  to  separate  sample  from  reject.  The  intermittent 
samplers,  as  the  name  implies,  deflect  the  entire  stream  at  in- 
tervals to  make  the  sample.  This  is  accomplished  by  passing  a 
sample  cutter  directly  across  the  stream. 

The  continuous  method  of  sampling  is  open  to  the  objection 
that  it  is  impossible  to  get  a  stream  of  falling  ore  containing  coarse 
and  fine  particles  which  is  uniform  across  its  entire  section.  This 
is  because  the  ore  on  its  way  from  the  preceding  crusher,  bin  or 
elevator  practically  always  passes  through  a  sloping  chute  in 
which  the  large  lumps  roll  away  from  the  small  ones  and  the  heavy 
minerals  become  more  or  less  separated  from  the  lighter  ones. 
Therefore,  any  continuously  taken  sample  will  be  either  richer  or 
poorer  than  the  average.  Because  of  these  conditions  this  type 
of  sampler  will  not  give  reliable  results,  and  is  now  but  little  used. 


ORE  SAMPLING 


55 


The  intermittent  method  of  sampling  gives  better  results.  The 
machine  should  be  so  designed  that  it  takes  equal  portions  all 
across  the  stream  at  frequent  and  regular  intervals.  In  one  mill 
the  first  time-sampler  cuts  24  and  the  last  one  76  sections  per 
minute. 

While  it  is  not  possible  to  produce  a  stream  of  ore  which  is 
uniform  in  value  throughout  its  entire  length,  and  no  single  cut 
would  be  likely  to  give  an  exact  representation  of  the  lot,  yet  if 
a  large  number  of  small ,  samples  be  taken  entirely  across  the 


FIG.  28.  —  Brunton  Time  Sampler. 

stream,  the  composite  thus  obtained  will,  according  to  the  theory 
of  probability,  approach  very  close  to  the  composition  of  the  entire 
lot.  It  is  essential  that  the  percentage  of  sample  taken  from 
all  parts  of  the  delivery  pipe  be  the  same ;  in  other  words,  that  the 
vertical  sample  section,  taken  in  a  direction  parallel  to  the  motion 
of  the  intake-spout,  should  be  a  rhomboid. 

Three  machines  of  this  type  have  come  into  general  use;  these 
are  the  Brunton,  the  Vezin  and  the  Snyder. 

Figure  28  is  a  line  drawing  of  the  Brunton  Time  Sampler.  It 
consists  of  an  oscillating  divider  swinging  back  and  forth,  in  a 
vertical  plane,  beneath  the  end  of  the  feed  spout.  It  is  suspended 
on  a  horizontal  shaft  and  swings  through  an  arc  of  120°.  It  re- 
ceives its  motion  through  a  train  of  gears,  a  disc  crank  and  rocker 


56 


A   TEXTBOOK  OF  FIRE  ASSAYING 


arm,  and  by  a  change  of  gears  any  proportion  of  the  stream,  from 
5  to  20  per  cent,  may  be  taken. 

The  sample  cutter,  which  has  horizontal  edges  not  shown  in 
the  figure,  deflects  the  sample  forward  into  a  hopper.     The  rest 


FIG.  29.  —  Vezin  sampler. 

of  the  ore  is  deflected  in  the  opposite  direction  into  a  chute  leading 
to  the  reject  bin.  It  is  essential  that  the  sample  cutter  move 
entirely  out  of  the  stream  in  each  direction  and  that  its  velocity 
be  uniform  while  any  part  of  it  is  underneath  the  falling  stream. 


ORE  SAMPLING  57 

Otherwise,  it  would  take  too  much  from  one  part  of  the  stream  and 
not  enough  from  other  parts. 

This  type  of  sampler  requires  less  head-room  than  any  of  the 
others  and  thus,  by  reducing  the  necessary  height  of  building, 
saves  in  the  cost  of  mill  construction.  Its  rocking  motion  helps 
to  dislodge  any  rags  or  strings  which  may  have  fallen  on  the  cut- 
ting edges  and  its  short  cutting  edges  render  accidental  distortion 
impossible.  A  further  advantage  claimed  for  the  Brunton  ma- 


FIG.  30.  —  Snyder  sampler. 

chine  is  that  centrifugal  force  assists  in  the  discharge  of  ore  from 
the  sampler  and  the  machine  can,  therefore,  be  run  at  a  much 
higher  rate  of  speed  than  any  of  the  sector  machines. 

The  Vezin  sampler,  shown  in  Fig.  29,  is  probably  the  best- 
known  automatic  sampler.  Various  modifications  in  shape  are 
possible,  but  generally  speaking,  it  consists  of  one  or  two  sample 
cutters  which  rotate  about  a  vertical  shaft  and  pass  through  a 
falling  stream  of  ore,  taking  out  a  part  of  it  and  conveying  this 
part  through  a  central  spout  to  a  sample  hopper.  The  theory  of 
sampler  design  requires  that  the  horizontal  cutting  edges  be  radii 


58  A   TEXTBOOK  OF  FIRE  ASSAYING 

of  the  axis  of  revolution.  This  is  necessary  in  order  to  ensure 
taking  the  same  amount  of  ore  from  every  part  of  the  stream. 
The  entire  mechanism  is  supported  in  a  frame,  the  bottom  of 
which  forms  a  hopper  to  collect  the  reject. 

Particularly  in  the  case  of  coarse  ore,  this  sampler  requires  more 
head-room  than  the  Brunton  and  its  long  cutting  edges  are  liable 
to  be  distorted.  It  is  designed  to  take  a  definite  proportion,  usu- 
ally one-fifth  of  the  stream,  and  this  proportion  cannot  be  altered 
after  the  machine  is  made. 

The  Snyder  sampler,  shown  in  Fig.  30,  is  the  simplest  of  all.  It 
consists  of  a  circular  casting  much  the  shape  of  miner's  gold-pan, 
mounted  on  the  end  of  a  horizontal  shaft.  One  or  more  holes 
are  made  in  its  sloping  flange  and  the  edges  of  these  project 
both  on  the  front  and  back  sides  of  the  flange.  The  sampler  re- 
volves from  ten  to  thirty  times  a  minute  and  the  material  to  be 
sampled  comes  to  it  by  way  of  a  sloping  chute,  not  shown  in  the 
cut.  The  ore  stream  falls  on  the  inside  of  the  sloping  flange  and 
either  passes  through  the  opening  into  a  suitable  sample  hopper 
or  slides  off  the  flange  into  a  hopper  leading  to  the  reject  chute. 

The  sides  of  the  sample  spout  should  lie  in  planes  passing  through 
the  axis  of  revolution.  Such  a  sampler,  60  inches  in  diameter, 
will  take  material  3|  inches  in  diameter. 

Figure  31  is  a  section  through  a  sampling  mill  and  shows  how  a 
number  of  crushers  and  samplers  are  combined  in  an  automatic 
plant.  To  simplify  the  drawing,  the  roll  feeders  have  been 
omitted.  Such  a  plant  will  treat  a  50-ton  carload  in  less  than  an 
hour.  It  is  cleaned  by  brushing  with  the  aid  of  compressed  air. 

Hand  and  Machine  Sampling  Compared.  —  In  comparing 
hand  and  machine  sampling  it  may  be  said  that  machine  sampling 
is  generally  cheaper,  and,  with  a  properly  designed  machine, 
is  more  accurate  than  coning  or  fractional  shoveling.  Perhaps 
the  most  important  advantage  of  all  is  that,  being  strictly  mechan- 
ical in  operation,  it  affords  less  opportunity  for  manipulation  of 
the  sample. 

Precaution  to  be  Observed.  —  Besides  the  danger  of  "salting  " 
from  crushing  machines,  elevators,  sampling  machines  etc.,  spe- 
cial attention  must  be  paid  to  the  disposition  of  the  fine  ore  dust. 
As  a  rule  the  rich  minerals  in  the  ore  are  more  brittle  than  the 
gangue,  with  the  result  that  the  ore  dust  is  far  higher  in  grade 
than  the  average  of  the  ore.  Whence  is  seen  the  necessity  of 


ORE  SAMPLING 


59 


96%  Discard 
3.2$  Discard-4, 
99..-2%  Discard 
RECEIVING  TRACK 


Sample  Safe 
O.I 6%  Sample 
No,  4  Sampler 
20&  Sample 


1st  CUT-400-LB. 
ZAUPLE  FROM  I  TON 

.  /  Sampler 
20%  SampJe 

COARSE 

CRUSHING  ROLLS 
16  x  36  Soils 

No.  2  Sampler 
20%  Sample 

2nd  CUT-80-LB. 
SAMPLE  FROM  I  TON 

FINE 

CRUSHING  ROLLS 
/4  x  27  Rolls 

3rd  CUT-I6-LB. 
SAMPLE  FROM  I  TO.N 
•No.  3  Sampler 
20%  Sample 

SAMPLE  ROLLS 

-f2  x  £0  RotJs 

Ufre  Shaft 

„      4Z7)   CLTT-3.2-LB. 
\          SAMPLE  FROM 
L  I  TON 


FIG.  31.  —  Taylor  and  Brunton  sampling  system. 


60  A   TEXTBOOK  OF  FIRE  ASSAYING 

preserving  all  of  the  ore  dust  and  of  taking  pains  to  see  that  the 
sample  contains  its  proper  proportion  of  the  same. 

Grab-Sampling.  —  This  is  a  rapid  method  used  for  sampling 
large  quantities  of  low-grade  and  uniform  material,  such  as  iron 
or  coal.  It  may  also  be  used  to  obtain  rough  samples  of  the  less 
homogeneous  ores  containing  copper,  lead,  zinc,  gold  and  silver. 
The  methods  of  sampling  iron  ore  and  coal  are  fairly  w^ll  standard- 
ized and  consist  in  taking  small  shovelsful  from  definite  points  in 
the  car  or  vessel  as  the  material  is  being  unloaded.  These  are 
combined  and  worked  down  by  some  of  the  standard  finishing 
methods. 

The  method  is  obviously  both  rapid  and  inexpensive,  but  is 
so  unscientific  that  no  one  considers  it  suitable  for  obtaining  a 
sample  from  which  the  amount  of  gold,  silver,  copper  or  lead 
contained  in  an  ore  is  to  be  determined.  Unfortunately,  how- 
ever, some  smelters  still  continue  to  use  the  grab-sample  to  deter- 
mine the  amount  of  moisture  in  custom  ores. 

Moisture  Sample.  —  Assays  and  chemical  determinations  are 
always  made  on  dry  samples  and  the  value  of  a  lot  of  ore  is  always 
figured  on  the  moisture-free  basis.  Except  in  cases  when  the 
entire  lot  may  be  dried,  it  is  necessary  to  take  a  sample  from  which 
to  determine  the  moisture.  This  sample  must  be  taken  as  quickly 
as  possible  after  the  ore  is  weighed.  If,  as  is  still  too  often  the 
case,  a  grab-sample  is  used  as  the  basis  for  a  moisture  determina- 
tion, much  of  the  careful  work  of  obtaining  the  sample  for  the  de- 
termination of  the  other  constituents  may  be  nullified.  Inasmuch 
as  fines  will  ordinarily  contain  very  much  more  moisture  than 
lump  ore,  and  as  the  grab-sample  is  small  in  amount,  it  is  clear 
that  any  sample  of  mine-run  ore  thus  taken  will  tend  to  carry 
more  than  its  due  share  of  moisture.  Such  a  result  leads  to  an 
undervaluation  of  the  ore,  due  to  the  fact  that  the  net  weight 
reported  is  too  small.  In  this  connection  it  should  not  be  for- 
gotten that  this  so-called  sample  is  taken  by  an  interested  party, 
an  employee  of  the  smelting  company,  who  may  be  entirely 
honest  but  who  certainly  will  not  purposely  lean  over  backwards 
in  his  efforts  to  be  fair  to  the  shipper.  At  any  event,  it  is  safe 
to  say  that  samples  for  the  determination  of  moisture  should  be 
taken  with  the  same  amount  of  care  as  samples  for  the  determina- 
tion of  metallic  contents,  and  that  apparently  the  simplest  and 
only  scientific  way  of  obtaining  them  from  shipments  of  mine-run 


ORE  SAMPLING  61 

ore  is  to  take  ~ them  from  the  sample  safe  or  reject  from  the  last 
mechanical  sampler. 

Since  the  ore  is  weighed  on  the  cars  before  it  is  sampled,  and 
since  in  dry  climates  there  is  obviously  some  loss  of  moisture  by 
evaporation  from  the  ore  in  its  passage  through  the  crushing, 
elevating  and  sampling  machinery,  it  is  customary  to  make  a 
correction  to  the  moisture-figure,  as  determined  by  this  latter 
method,  to  compensate  for  this  loss.  This  correction  usually 
consists  of  the  addition  of  an  arbitrary  percentage.  Brunton* 
finds  10  per  cent  in  summer  and  7  per  cent  in  winter  a  fair  average 
figure.  For  instance,  if  the  sample  showed  5  per  cent  moisture 
for  a  lot  of  ore  shipped  during  the  summer  months  a  fair  figure 
for  the  actual  moisture  content  would  be  5.5  per  cent. 

This  addition  of  a  more  or  less  arbitrary  correction  is  not  en- 
tirely satisfactory  and  the  reason  for  it  is  not  always  understood 
by  the  shippers,  but  in  spite  of  this  practice  the  latter  method  of 
arriving  at  the  moisture  content  of  an  ore  is  far  superior  to  that 
which  depends  on  the  grab-sample,  and  with  its  use  there  are  fewer 
disputes  and  less  ill-feeling  between  seller  and  buyer. 

Moisture  determinations  are  made  in  duplicate  on  samples 
weighing  from  2  to  5  pounds.  These  are  weighed  out  into  por- 
celain or  enameled-iron  dishes  and  dried  at  105°  C.,  the  loss  of 
weight  being  called  moisture. 

The  moisture-figure,  either  because  of  the  method  of  taking  the 
sample,  or  the  amount  of  the  compensating  correction  applied, 
still  continues  to  be  a  frequent  source  of  dissatisfaction  on  the 
part  of  sellers  of  ore.  The  practice  of  taking  one  or  more  grab- 
samples  from  each  car  of  ore  is  the  most  unscientific  part  of  the 
whole  ore-purchasing  business.  This  practice  is  unfortunately 
still  in  common  use  even  when  the  ore  is  of  such  a  nature  that  it 
must  be  passed  through  a  mechanical  sampling  plant  to  obtain 
the  sample  used  for  determining  the  metallic  contents.  In  this 
latter  case,  grab-sampling  has  nothing  to  recommend  it,  unless 
it  be  the  opportunity  for  manipulation  or  error,  and  it  should  be 
abandoned.  In  case  the  ore  is  of  such  a  nature  that  a  satis- 
factory sample  for  the  determination  of  metallic  contents  can  be 
obtained  without  mechanical  sampling,  the  same  method  may  or- 
dinarily be  applied  to  obtain  a  moisture  sample. 

*  Trans.  A.I.M.E.  40,  p.  567  (1909). 


62 


A   TEXTBOOK  OF  FIRE  ASSAYING 


Duplicate  Sampling.  —  To  check  the  accuracy  of  the  sampling 
operations,  we  may  resort  to  the  process  of  duplicate  sampling 
or  to  resampling.  Duplicate  sampling  in  the  laboratory  should 
consist  in  first  cutting  the  entire  lot  into  two  portions  and  then 
sampling  each  one  separately.  As  a  general  rule,  the  results 
should  check  within  1  per  cent.  If  they  do  not,  it  indicates 
either  poor  mixing  and  cutting  or  a  too  rapid  reduction  of  sample. 

Some  sampling  mills  are  arranged  to  allow  for  taking  duplicate 
samples,  so  that  they  have  constant  checks  on  the  accuracy  of 
their  sampling  operations.  The  following  results  of  assays  made 
on  original  and  resampled  lots  are  taken  from  D'.  K.  Brunton's 
paper  on  "  Modern  Practice  of  Ore-Sampling  "  in  the  Transac- 
tions of  the  American  Institute  of  Mining  Engineers*  and  shows 
how  closely  such  work  is  made  to  check. 

TABLE  VII. 
RESULTS  OF  RESAMPLING. 


Lot  No. 

Sample 
Ounces  gold  per  ton 

Resample 
Ounces  gold  per  ton 

3192 

3.62 

3.64 

3198 

5.04 

5.015 

3219 

2.70 

2.67 

3235 

3.18 

3.16 

3310 

1.17 

1.17 

3324 

6.52 

6.51 

3340 

0.71 

0.78 

3388 

1.70 

1.84 

3424 

9.24 

9.20 

3471 

30.64 

30.52 

FINISHING  THE   SAMPLE. 

The  12-  or  14-mesh  ore  cut  by  the  last  sampler  is  further  re- 
duced in  size  by  the  use  of  sample  grinders,  and  its  weight  is 
reduced  by  coning  and  quartering  or  by  riffle  cutting.  The 
principles  of  sampling  laid  down  in  the  first  part  of  the  chapter 
should  be  followed  throughout,  even  to  the  final  portion  which 
is  weighed  out  for  assay  determination. 

As  the  sample  grows  smaller  more  and  more  care  has  to  be  taken 
to  prevent  contamination  or  "salting."  A  few  particles  of  rich 
ore,  which  if  introduced  into  the  original  lot  would  have  had  no 
*  Trans.  A.I.M.E.  40,  p.  567  (1909). 


ORE  SAMPLING  63 

material  effect  on  the  average,  might  seriously  alter  the  result 
if  allowed  to  enter  the  final  sample. 

Before  the  final  pulverizing  is  begun,  the  sample  should  be  thor- 
oughly dried  by  heating  to  100°  or  110°  C.  No  greater  degree  of 
heat  than  this  should  be  used,  as  there  is  danger  of  roasting  the 
sulphides  or  otherwise  altering  the  composition  of  the  ore. 

Mixing.  —  In  addition  to  coning,  the  following  methods  of 
mixing  are  frequently  used  in  some  stage  of  the  finishing  treat- 
ment of  ore. 

1.  ROLLING.     For  lots  of   100  pounds  or  less  the  method  of 
mixing,  .whereby  the  ore  is  rolled  on  canvas,  rubber  sheeting  or 
paper,  is  often  used.     When  the  ore  particles  are  fairly  uniform 
in  size  and  specific  gravity,  this  method  is  satisfactory,  but  for 
ordinary  ores  in  the  coarse  state,  it  should  be  avoided.     For 
ore  crushed  so  fine  that  it  has  little  or  no  tendency  to  stratify,  as 
for  example  the  assay  pulp  ground  to  100-  or  120-mesh,  the  method 
is  found  satisfactory  when  the  operation  is  properly  performed. 
This  method  is  almost  universally  used  by  assayers  for  mixing 
the  final  lot  of  pulverized  ore,  just  before  taking  out  the  assay 
portion. 

2.  POURING.     For  small  samples  the  method  of  pouring  from 
one  pan  into  another  is  sometimes  employed,   especially  as   a 
preliminary  to  riffle  cutting.     Like  the  one  above,  it  is  imperfect 
when  performed  on  an  ordinary  mixture  of  coarse  and  fine  ore. 

3.  SIFTING.     For  mixing  small  lots  of  ore  or  fluxes,  the  method 
of  sifting  is  particularly  good.     The  apertures  in  the  sieve  should 
be  two  or  three  times  as  large  as  the  largest  particles.     The  ore 
should  be  placed  on  the  sieve  a  little  at  a  time  and  allowed  to  fall 
undisturbed  into  a  flat  receiving  pan,  until  all  the  ore  has  passed 
the  sieve.     Two  or  three  siftings  are  equivalent  to  100  rollings. 
Sifting  has  the  further  advantage  over   the  other  methods  that 
all  lumps  are  broken  up  and  the  ore  composing  them  distributed. 
It  should  be  noted   that   sifting  with  a  screen,  the  apertures  of 
which  are  smaller  than  the  coarsest  particles  of  Ore,  will  tend  to 
separate  hard  and   tough  minerals  which  resist  grinding,  from 
soft  and  brittle  ones  which  tend  to  become  very  finely  pulverized. 

Grinding.  —  Two  kinds  of  grinders  are  used  for  finishing  work, 
the  cone-grinders  and  the  disc-pulverizers.  They  should  be  so 
constructed  that  they  may  be  easily  and  thoroughly  cleaned. 
Many  excellent  pulverizers  are  unsuited  for  sampling  work  on 


A   TEXTBOOK  OF  FIRE  ASSAYING 


FIG.  32.  —  Disc  pulverizer  closed 


FIG.  33.  —  Disc  pulverizer  open  for  cleaning. 


ORE  SAMPLING  65 

account  of  the  labor  and  difficulty  involved  in  cleaning  them 
effectively. 

Figure  32  shows  a  thoroughly  reliable  and  efficient  disc-pulver- 
izer which  takes  J-inch  ore  and  reduces  it  in  one  operation  to 
100-mesh  or  finer.  It  is  as  nearly  dust-tight  as  possible,  and  the 
grinding  plates  are  renewable.  It  is  shown  open  for  cleaning 
in  Fig.  33. 

The  bucking-board  is  now  but  little  used  for  grinding  ores  ex- 
cept in  the  case  of  very  small  samples  weighing  less  than  100 
grams  where,  because  of  ease  of  cleaning  and  small  dust  loss, 
it  may  still  be  used.  It  is  also  used  to  regrind  the  last  oversize 
resulting  from  screening. 

One  of  the  best  methods  of  cleaning  the  bucking-board  or 
sample-grinder  is  to  brush  it  out,  then  grind  a  quantity  of  some 
barren  material,  such  as  sand  or  crushed  fire-brick,  and  follow 
this  by  a  second  brushing. 

Screening.  —  It  is  customary  in  careful  work  to  screen  all 
final  samples  of  assay  pulps.  Although  a  good  pulverizer,  prop- 
erly adjusted,  will  grind  practically  everything  fine  enough  to  pass 
100-  or  120-mesh  in  one  pass,  the  exact  adjustment  is  difficult  to 
get  and  to  maintain  on  account  of  wear  and  expansion  due  to 
heating.  Besides,  there  is  always  a  small  amount  of  ore  remain- 
ing in  the  feed  chute,  which  has  not  been  ground  and  which  in 
itself  necessitates  screening  of  the  pulp.  The  screens  should  be 
at  least  9  inches  in  diameter  to  give  satisfactory  capacity,  and 
the  screen  wire  should  be  of  uniform  grade.  The  screen  itself 
consists  of  a  suitable  frame  in  which  the  screen  wire  is  stretched, 
fitting  into  a  pan  which  holds  the  sifted  ore.  They  should  both 
be  free  from  crevices  which  might  provide  lodging  places  for  ore, 
which  would  be  given  up  later  to  enrich  a  subsequent  sample. 

The  operation  of  screening  consists  of  a  combination  of  shak- 
ing in  a  horizontal  plane  and  tapping  of  the  screen  against  the 
table-top  or  work-bench  to  keep  the  meshes  clean.  In  most 
cases  it  is  neither  necessary  nor  desirable  to  use  washers  or  a 
brush  to  assist  in  screening.  They  both  tend  to  force  oversize 
particles  through  the  screen.  Screens  should  be  carefully  brushed 
out  after  sifting  each  sample,  and  after  a  high-grade  ore  has  been 
screened  some  of  the  barren  material  put  through  to  clean  the 
grinder  should  be  sifted  and  then  thrown  away. 

The  sifted  ore  should  be  thoroughly  mixed  before  sampling, 


66  A   TEXTBOOK  OF  FIRE  ASSAYING 

as  screening  under  these  conditions  favors  a  certain  amount  of 
segregation. 

Size  of  Assay  Pulp.  —  For  assay  purposes,  all  ore  should  be 
reduced  to  at  least  100-mesh  and  rich  spotty  ores  should  be  pul- 
verized to  120-  or  140-mesh  or  finer  to  ensure  a  fair  sample  being 
obtained  for  the  final  crucible  or  scorification  assay.  For  a  cru- 
cible assay  using  1  assay-ton,  an  ore  may  be  left  coarser  than  for 
a  scorification  assay  where  only  0.1  assay-ton  charge  is  used, 
If  the  assayer  has  difficulty  in  obtaining  results  checking  within 
one-half  of  one  per  cent  he  may  well  look  for  the  difficulty  in  the 
size  of  the  assay  pulp.  Very  often  a  regrinding  to  a  finer  size 
will  overcome  the  difficulty. 

When  any  portion  of  ore  has  been  selected  as  a  sample  and  is 
to  be  passed  through  a  sieve,  it  is  essential  that  the  whole  sample 
be  made  to  pass.  The  harder  portions  which  resist  crushing 
the  longest  are  almost  invariably  of  a  different  composition  from 
the  remainder  and  if  rejected  render  the  whole  sample  worthless. 

ORES   CONTAINING   MALLEABLE   MINERALS. 

In  the  crushing  of  ores  containing  native  gold,  silver  and  cop- 
per also  chloride,  bromide,  iodide,  or  sulphide  of  silver,  as  well 
as  other  malleable  minerals,  more  or  less  of  these  will  be  left  on 
the^sieve  as  flat  scales,  cylinders  or  spheres.  When  an  ore  which 
might  be  expected  to  contain  such  minerals,  is  being  sampled  great 
care  should  be  observed,  first,  in  watching  for  the  metallics  and  see- 
.ing  that  they  are  saved,  as  the  inexperienced  operator  is  likely 
not  to  appreciate  their  value  and  to  throw  them  away,  and 
second,  to  so  conduct  the  grinding  that  they  may  be  removed 
at  every  opportunity.  The  coarser  these  particles  are  the  more 
difficult  it  becomes  to  obtain  an  accurate  sample,  both  from  the 
standpoint  of  sampling  theory  and  from  the  fact  that  a  larger 
amount  of  highly  intelligent  and  painstaking  labor  is  necessary 
throughout  each  stage  of  the  sampling  and  assaying  process. 

Two  mistakes  are  common.  The  first  is  to  throw  away  a 
small  amount  of  residue  resting  on  the  screen  without  carefully 
examining  it  to  ensure  the  absence  of  any  valuable  constituent. 
The  second  arises  from  the  practice,  occasionally  noticed,  of 
putting  the  metallics  back  on  the  bucking-board  or  into  the 
grinding  machine  with  a  small  amount  of  the  pulverized  ore  and 
continuing  the  grinding  until  everything  passes  the  sieve.  This 


ORE  SAMPLING  67 

latter  practice  is  fully  as  objectionable  as  the  first,  both  on  account 
of  the  impossibility  of  obtaining  an  even  distribution  of  the 
metallic  particles  in  the  final  sample  of  assay  pulp  weighed  into 
the  crucible  and  because  of  the  loss  resulting  from  the  smearing 
of  the  me  tallies  on  the  working  surfaces  of  the  grinding  machines. 
A  secondary  disadvantage  of  this  latter  practice  is  the  danger  of 
salting  the  next  sample  from  the  metal  remaining  on  the  grind- 
ing surfaces,  particularly  if  the  sample  is  low-grade.  Lodge* 
gives  an  example  of  this  kind  to  illustrate  the  necessity  of  a  thor- 
ough cleaning  of  machines  and  bucking-boards  after  rich  ore 
has  been  ground.  In  this  case  sand  carrying  0.04  ounce  per 
ton  in  gold,  after  grinding  in  a  "  salted  "  machine,  was  found  to 
assay  0.78  ounce  per  ton. 

When  an  ore  containing  metallics  is  being  sampled  the  original 
sample  must  be  carefully  weighed,  the  particles  found  on  each  sieve 
must  be  separately  preserved  and  weighed,  and  the  pulp  result- 
ing from  each  sampling  and  sifting  must  also  be  weighed.  This 
not  only  gives  the  data  from  which  to  calculate  the  true  or  "  me- 
tallic "  assay  of  the  sample  submitted  but  also  acts  as  a  check  on 
any  carelessness  in  the  whole  sampling  operation.  If  the  pellets 
are  gold  or  silver  they  are  wrapped  in  lead  foil,  cupeled,  weighed 
and  parted.  If  of  copper,  as  in  the  case  of  an  ore  containing  na- 
tive copper,  the  weight  of  the  metallic  contents  is  otherwise 
established,  perhaps  by  cleaning  in  hydrochloric  acid  and  direct 
weighing,  or  by  making  a  fusion  as  in  the  Lake  Superior  fire- 
assay.  Other  cases  may  arise ;  for  instance,  in  the  sampling  of 
molybdenum  ores  flat  scales  of  molybdenite  left  on  the  screen 
will  require  special  attention.  Various  metallurgical  products, 
as  for  instance,  slag,  matte,  furnace  or  cupel  bottom,  dross, 
litharge,  precipitate,  etc.,  very  often  contain  metallics  and  must 
be  handled  in  this  way. 

Calculation  of  Results.  —  Various  writers  give  rules  and  for- 
mulas both  for  assaying  and  calculating  results  of  this  sort.  No 
simple  formula  can  cover  all  cases  and  no  rule  nor  formula  can 
take  the  place  of  the  experience  and  common  sense  of  the  practi- 
cal assayer,  so  that  each  example  should  be  made  an  individual 
problem  with  its  proper  assumptions  based  on  actual  knowledge 
of  conditions  and  occurrences  during  the  sampling.  The  follow- 
ing example  obtained  in  the  assay  of  an  ore  from  Cobalt,  Ontario, 
*  Notes  on  Assaying,  p.  32. 


68  A   TEXTBOOK  OF  FIRE  ASSAYING 

will  illustrate  some  of  the  problems  which  have  to  be  taken  into 
consideration. 

DATA.  —  Dry  weight  of  sample  received  129.6  grams.  Size  16- 
mesh.  This  was  crushed  to  pass  a  120-mesh  screen  and  yielded 
metallics  on  the  screen  5.60  grams  and  pulp  through  the  screen 
121.6  grams.  The  pellets  were  scorified  and  cupeled,  giving  3.823 
grams  of  silver.  The  pulp  assayed  1992  ounces  per  ton  silver. 

SOLUTION.  —  It  is  at  once  seen  that  the  sum  of  the  weight  of  the 
pellets  and  pulp  do  not  equal  the  weight  of  the  original  sample, 
the  difference  or  loss  being  2.4  grams.  The  assayer  must  decide 
what  to  with  this  loss  before  proceeding  to  calculate  the  assay. 
However,  it  should  first  be  pointed  out  that  some  loss  is  inevitable, 
the  dust  in  any  grinding  room  being  sufficient  evidence  of  this. 
The  assayer  does  not  know  how  much  metal  value  this  lost  ore 
actually  carries,  but  he  knows  that  it  does  carry  some  and  for 
that  reason  he  cannot  neglect  it.  It  is  obvious,  however,  that 
he  should  observe  every  precaution  to  keep  the  loss  at  a  minimum 
to  reduce  this  uncertain  factor.  In  the  present  case  it  was  be- 
lieved that  all  of  the  loss  was  in  dust,  which  was  assumed  to  assay 
the  same  as  the  fine  ore  pulp.  The  amount  of  silver  in  129.6  - 
5.6  =  124.0  grams  of  pulp,  assaying  1992  ounces  per  ton,  is 
then  calculated  and  added  to  the  silver  from  the  pellets.  This 
then  gives  the  total  silver  contained  in  the  original  sample  of  129.6 
grams. 

Silver  in  metallics  3 . 823  grams 

1 94.  0  V    1  QQ9 
Silver  in  124.0  grams  of  pulp         "*  *      -   =    8.469      " 


Total  silver  in  sample  12 . 292     " 

The  average  amount  of  silver  in  one  assay-ton  of  the  original 
sample  would  then  be  found  from  the  following  proportion: 
129.6  :  12.29  =  29.166  :  x 

When  this  is  solved  x  is  found  to  be  2.765  grams.  Whence  the 
"  metallic  "  assay  of  the  ore  is  2765  ounces  silver  per  ton. 

In  order  to  show  the  method  clearly,  the  above  calculation  has 
been  worked  out  with  more  precision  than  is  ordinarily  necessary. 
Whether  the  sample  of  129.6  grams  of  16-mesh  ore  is  a  reliable  one 
is  open  to  question,  but  it  is  obviously  the  duty  of  the  assayer  to 
analyze  the  material  submitted  to  him  to  the  best  of  his  ability, 
regardless  of  the  above  consideration. 


ORE  SAMPLING 


69 


There  may  be  a  shorter  method  of  calculating  the  "  metallic  " 
assay  in  the  simple  case  shown  above,  but  in  the  more  compli- 
cated cases,  where  metallics  are  found  on  several  screens  in  the 
reduction  of  a  sample  of  considerable  size,  it  is  best  to  follow  the 
general  method  illustrated  as  being  less  likely  to  lead  to  confusion 
and  error.  One  additional  assumption  has  to  be  made  when  a  lot 
of  ore  reduced  by  stages  yields  metallics  on  different  screens: 
in  any  sampling  the  reject  contains  the  same  proportionate  weight 
and  value  in  pellets  as  the  sample.  It  need  hardly  be  mentioned 
that  if  the  proper  ratio  between  size  of  sample  and  maximum  grain 
has  been  maintained,  the  above  assumption  will  be  borne  out  in 
practice. 

The  following  example  illustrates  the  more  complicated  case: 

CALCULATION  OF  ASSAY  WHEN  ORE  CONTAINS  COARSE 

PARTICLES  OF  NATIVE  GOLD. 

DATA 


A  sample  of  23.75  kilo- 
grams or  23,750  grams 
was  crushed  to  pass  a 
40-mesh  sieve. 


On  sieve  25  grams. 
This  yielded  6.2750 
grams  of  gold. 

Through  sieve  23,600 
grams  (Loss  125  grams). 

A  sample  from  this  of 
5825  grams  was  crushed 
to  pass  a  120-mesh  sieve. 


On  sieve  3  grams.  This 
yielded  1.6720  grams  of 
gold. 

Through  sieve  5802 
grams  (Loss  20  grams). 

The  fine  ore  assays 
1.21  ounces  gold  per  ton. 


CALCULATIONS. 

Total  pellets  from  23,750  grams  of  ore  on  40- 
mesh 

Total  40-mesh  ore  assuming  loss  to  be  same  as 
the  rest,  i.e.,  sample  now  23,725  grams. 

Total  pellets  from  23,725  grams  on  120-mesh 
23,725 


5825 


X  1.6720  = 


Weight  Gold 
6 . 275  grams 

6.810    " 


Assuming  all  of  ore  to  be  crushed  through  120- 
mesh  and  no  loss,  there  would  be  23,725 

X  3  =  23,713  grams  fine  ore  (as- 


saying  1.21  ounces) 


Total  gold  in  this  = 


23,713  X  .00121 
29.166 


Total  gold  in  original  lot 


70  A   TEXTBOOK  OF  FIRE   ASSAYING 

29.166  :  x  =  23,750  :  14.066 

x  =  .01727  =  gold  from  1  assay- ton 
Ore  assays  17.27  ounces  per  ton. 

If  the  metallics  are  mainly  iron  or  other  barren  material  the 
metallic  assay  may  be  lower  rather  than  higher  than  the  assay 
of  the  fine  pulp. 


.      CHAPTER  IV. 
BALANCES  AND   WEIGHTS. 

The  reliability  of  every  assay  or  other  quantitative  determi- 
nation is  directly  dependent  upon  the  accuracy  of  the  weighing, 
both  of  the  ore  charge  and  more  especially  of  the  resultant  product, 
for  example,  the  silver  bead  or  the  parted  gold.  Any  error  made 
in  the  weighing  will,  of  course,  invalidate  all  the  rest  of  the  work, 
regardless  of  the  care  which  may  have  been  given  it.  The  oper- 
ator should,  therefore,  familiarize  himself  with  the  construction, 
sensitiveness  and  operation  of  his  balance  before  he  attempts  to 
do  any  accurate  assaying. 

A  good  assay  balance,  used  carefully  and  intelligently  is  capable 
of  weighing  to  0.01  milligram  or  0.00001  gram.  For  the  most 
delicate  assay  balances  an  accuracy  of  0.000002  gram  is  claimed. 
The  necessity  of  weighing  to  this  degree  of  accuracy  may  be  under- 
stood when  it  is  considered  that  if  the  usual  charge  of  ore,  1  assay- 
ton,  is*  represented  by  a  sample  of  29.166  grams  or  about  an  ounce, 
and  the  resultant  gold  is  weighed  to  the  nearest  0.01  milligram, 
the  value  of  the  ore  is  only  determined  to  within  20  cents  per  ton. 
This  is  usually  sufficiently  close,  but  any  less  degree  of  accuracy 
would  not  be  so  considered. 

At  least  three  grades  of  balances  are  necessary  for  the  fire- 
assay  laboratory.  These  are  known  as  flux,  pulp,  and  button  or 
assay  balances..  In  large  assay  laboratories,  there  are  also  usually 
found  bullion  and  chemical  balances  as  well  as  separate  assay 
balances  for  gold  and  for  silver. 

Flux  Balance.  —  The  flux  balance,  for  the  weighing  of  fluxes, 
reagents,  etc.,  should  be  an  even  balance  scale,  provided  with  a 
removable  scoop-shaped  pan,  capable  of  weighing  2 -kilograms  and 
sensitive  to  0.1  gram.  Figure  34  shows  a  most  satisfactory  flux 
balance  made  with  agate  bearings  and  side-beam  graduated  from 
0.1  to  5.0  grams.  With  this  balance  no  weight  smaller  than  5 
grams  is  required. 

71 


72  A  TEXTBOOK  OF  FIRE  ASSAYING 


FIG.  34.  —  Flux  balance 


FIG.  35.  —  Pulp  balance. 


BALANCES  AND  WEIGHTS  73 

Pulp  Balance.  —  The  pulp  balance  for  weighing  the  ore  or  pulp 
for  assay  and  the  buttons  from  lead  assays,  etc.,  should  be  an 
even  balance  scale.  The  pans  should  be  made  removable  and 
should  each  have  a  capacity  of  at  least  2  ounces  of  sand.  The 
pulp  balance  should  be  enclosed  in  a  glass  case  and  should  be 
sensitive  to  half  a  milligram.  Such  balances  are  sometimes  listed 
in  the  manufacturers'  catalogue  as  prescription  balances.  If  more 
than  one  pulp  balance  is  to  be  obtained,  it  is  well  to  get  one  or 
more  having  a  pan  capacity  of  4  or  5  ounces  of  sand.  For  one- 
half  and  1  assay-ton  charges  the  2  ounce  pan  is  to  be  preferred, 
as  it  is  easier  to  transfer  ore  from  it  to  the  crucible  than  from  a 
larger  pan.  Figure  35  shows  a  good  type  of  pulp  balance  made 
with  steel  edges  and  agate  bearings. 

Assay  Balance.  —  The  button  or  assay  balance  is  the  most  sen- 
sitive balance  made.  It  should  be  capable  of  weighing  to  at 
least  0.01  milligram,  should  be  rapid  in  action,  making  a  com- 
plete oscillation  in  from  10  to  15  seconds,  and  should  have  stabil- 
ity of  poise,  that  is  to  say  that  it  should  be  so  made  that  its  ad- 
justments will  not  change  sensibly  from  day  to  day  owing  to 
slight  changes  of  temperature  and  atmospheric  conditions.  The 
capacity  of  the  assay  balance  need  not  be  large,  0.5  gram  maxi- 
mum being  sufficient,  but  the  beam  should  be  rigid  at  this  load. 

Such  a  delicate  piece  of  apparatus  must  be  handled  with  great 
care  if  good  service  is  expected  of  it.  It  should  be  as  far  as  possi- 
ble from  any  laboratory  or  part  of  the  plant  where  corrosive  fumes 
are  being  evolved,  and  should  be  covered  when  not  in  use,  to  keep 
out  the  dust. 

The  balance  beam  should  be  as  light  as  it  can  be  made  with- 
out sacrificing  the  necessary  rigidity.  For  this  reason  the  truss 
frame  construction  is  usually  adopted,  giving  the  maximum 
strength  with  the  minimum  weight.  The  construction  should  be 
such  that  the  two  balance  arms  are  of  equal  weight  and  length, 
and  the  three  knife-edges  should  all  lie  in  the  same  plane.  The 
material  of  the  beam  should  be  non-magnetic  for  obvious  reasons, 
and  should  have  a  small  coefficient  of  expansion.  The  knife- 
edges  and  bearings  should  be  of  agate,  ground,  polished  and 
mounted  so  as  to  have  equal  angles  on  each  side.  The  knife- 
edges  should  be  so  sharp  that  a  strong  pocket-lens  will  show  no 
flatness  on  the  bearing  edge  and  the  agate  bearings  should  appear 
perfectly  smooth.  All  of  the  metal  work  of  the  balance  should 


74 


A   TEXTBOOK  OF  FIRE  ASSAYING 


be  protected  from  attack  by  chemical  fumes,  by  some  such  means 
as  gold-plating  or  lacquering.  Lacquering  seems  to  resist  chemi- 
cal fumes  rather  better  than  the  ordinary  gold-plating.  The  con- 
struction of  the  balance  should  be  such  that  the  rider  may  be  placed 
on  the  zero  graduation  and  used  from  the  zero  point  to  the  end 
of  the  beam. 


FIG.  36.  —  Gold  assay  balance. 


The  balance  must  be  mounted  in  such  a  way  that  it  will  be  free 
from  vibration.  Such  a  support  may  be  obtained  by  placing 
the  shelf,  on  which  the  balance  rests,  on  one  or  more  posts  which 
are  set  in  the  ground  and  which  come  up  through  the  floor  with- 
out touching  it. 

There  are  a  number  of  good  assay  balances,  many  of  them  pro- 
vided with  reading  glasses  and  other  special  attachments.  Figure 
36  shows  one  of  the  inverted  type,  the  principal  advantage  of 
which  is  that  when  the  pointer  is  inverted,  the  ivory  scale  is  on 
a  level  with  the  eye.  This  construction  necessitates  the  off- 


BALANCES  AND  WEIGHTS  75 

setting  of  the  zero  graduation  on  the  beam.  By  omitting  the 
graduations,  the  beam  may  be  made  lighter  and  is  not  subject 
to  strain  and  distortion  due  to  graduating  and  numbering.  The 
balance  shown  in  the  illustration  has  a  very  light  trussed-beam 
which  is  not  graduated.  The  beam  is  practically  invisible  in 
the  cut  but  its  reflection  in  the  glass  base-plate  is  quite  clear. 
A  white  scale  on  the  rider  bar  carries  the  graduations,  and  a 
pointer  attached  to  the  rider  arm  indicates  on  the  scale  the  exact 
position  of  the  rider  on  the  beam. 

Theory  of  the  Balance.  —  The  balance  is  essentially  a  light 
trussed  beam,  supported  at  its  center  by  a  knife-edge.  At  each 
end  of  the  beam  is  hung  a  scale-pan.  The  two  pans  should  be 
of  equal  weight. 

A' 

A 


M 

FIG.  37.  —  Line  drawing  of  balance. 

Let  the  three  knife-edges  A,  B,  and  C,  be  in  the  same  straight 
line.  Let  AB  =  BC  =  I.  Let  G  be  the  center  of  gravity  of 
the  beam,  whose  weight  is  W.  Let  the  distance  of  the  center  of 
gravity  below  the  point  of  support,  BG  =  V . 

With  a  load  of  M  in  each  pan  there  will  be  equilibrium.  Now 
if  a  small  weight  (m)  be  added  to  the  right-hand  pan,  the  balance 
will  swing  through  a  small  angle  9  and  the  beam  will  again 
come  to  equilibrium  in  a  new  position  A'BC'.  The  condition  for 
equilibrium  will  be  obtained  by  taking  the  moments  of  the  three 
forces,  M,  M  +  m  and  W  about  the  axis  B.  This  gives  the 
relation 

Ml  cos  9  +  V  sin  9  W  =  (M+  m)  I  cos  0 

sin  9  Im 

or =  tan  9  =  ^777. 

cos  0  Wl 

The  sensitiveness  of  a  balance  is  usually  denoted  by  the  angle 
through  which  the  beam  will  swing  when  a  small  weight,  usually 


76  A   TEXTBOOK  OF  FIRE  ASSAYING 

1  milligram,  (for  assay  balance  0.1  milligram)  is  added  to  one 
pan.  For  small  angles  the  tangent  and  its  angle  may  be  taken  as 
equal  and  therefore  the  expression  deduced  for  tangent  9  above 
may  be  taken  as  a  measure  of  the  sensitiveness  of  the  balance. 

The  equation  for  tangent  shows  that  the  sensitiveness  of  a 
balance  varies: 

(a)    Directly  as  the  length  of  the  balance  arms. 

(6)    Inversely  as  the  weight  of  the  beam. 

(c)    Inversely  as  the  distance  of  the  center  of  gravity  below 
the  point  of  support.     (Distance  BG.) 

The  sensitiveness  is  seen  to  be  independent  of  the  load  if  the 
three  knife-edges  are  in  the  same  straight  line,  and  most  balance 
makers  attempt  to  approach  this  condition  in  making  assay 
balances.  When  B  is  above  AC  the  sensitiveness  is  decreased 
with  the  load;  when  B  is  below  AC  it  is  increased  up  to  a  certain 
limit,  beyond  which  the  equilibrium  becomes  unstable. 

The  condition  of  uncreased  sensitiveness  with  long  beam  and 
small  weight  (a)  and  (6)  conflict,  as  the  longer  the  beam  is 
made  the  heavier  it  must  be  The  length  of  the  arm  is  also 
limited  by  the  time  of  swing  of  the  balance,  which  may  be  con- 
sidered to  be  a  compound  pendulum.  A  period  of  about  twelve 
or  fifteen  seconds  is  required  for  a  complete  oscillation.  Formerly 
the  long  arm  balances  were  common,  but  at  present  the  makers 
are  restricting  the  length  of  the  beam  to  5  inches. 

By  bringing  the  center  of  gravity  nearer  to  the  center  of  sup- 
port the  sensitivity  is  increased.  As  the  center  of  gravity  nears 
the  center  of  support,  the  stability  of  poise  decreases.  If  the 
two  should  coincide  there  would  be  no  point  of  rest  and  the  bal- 
ance would  be  unstable  or  "  cranky."  The  most  difficult  thing 
to  obtain  is  a  balance  with  great  stability  and  extreme  sensitivity. 
It  is  obtained  by  making  the  beam  as  light  as  possible  and  then 
keeping  the  center  of  gravity  sufficiently  below  the  center  knife- 
edge  to  give  the  necessary  stability.  Most  high-grade  balances 
are  provided  with  a  screw-ball  or  sliding-weight  so  that  the  center 
of  gravity  may  be  adjusted.  If  the  balance  lacks  stability,  i.  e., 
is  cranky  and  over-sensitive,  both  of  those  conditions  may  be 
remedied  by  lowering  this  weight  and  thus  lowering  the  center  of 
gravity  of  the  system. 

In  the  above  discussion  the  assumption  has  been  that  the  arms 
of  the  balance  were  equal.  Modern  high-grade  balances  usually 


BALANCES  AND  WEIGHTS  77 

approach  very  closely  to  this  condition.  The  process  of  "  double 
weighing  "  serves  to  eliminate,  however,  any  error  in  weighing 
that  may  be  due  to  inequality  of  the  arms.  Call  the  observed 
weight  of  the  body  as  weighed  in  pan  A,  W,  and  that  in  pan 
C,  W".  Then  W,  the  true  weight,  is  found  as  follows: 

W  =  VWW" 

W'  -f  W" 
when  W  and  W"  are  nearly  equal  W  =  -  — • 

General  Directions  for  Weighing.  —  Brush  off  the  pans  and  if 
necessary  clean  the  front  plate  of  the  balance.  See  that  the 
weighing  rider  is  on  the  zero  graduation  or  on  the  carrier,  as  the 
balance  may  require.  Adjust,  if  necessary,  to  make  the  point  of 
rest  coincide  with  the  center  graduation  on  the  ivory  scale  and 
try  the  adjustment  every  time  you  have  any  weighing  to  do, 
as  it  is  never  safe  to  assume  that  the  balance  will  stay  in  equilib- 
rium. Note  the  maximum  load  the  balance  will  carry  and  do 
not  exceed  this. 

Put  the  balance  into  action  by  gently  lowering  the  beam  onto 
the  knife-edges.  It  may  then  start  swinging  slightly  of  its  own 
accord.  If  it  does  not,  set  it  swinging  by  gently  fanning  one 
pan  with  a  motion  of  the  hand,  or  by  lifting  the  rider  for  an  in- 
stant and  then  putting  it  back  on  the  beam.  The  balance  may 
be  started  swinging  by  blowing  gently  on  one  pan  with  a  device 
such  as  a  medicine  dropper.  If  the  balance  is  started  swinging 
by  fanning  with  the  hand,  it  should  be  allowed  to  make  one  or 
two  complete  oscillations  before  a  reading  is  taken,  to  prevent 
air  currents  from  interfering  with  the  normal  swing.  Have  the 
amplitude  of  swing  not  more  than  1  or  2  divisions  each  side  of  the 
center. 

In  reading  the  position  of  the  pointer  on  the  ivory  scale,  arrange 
always  to  have  the  reading  eye  in  the  same  position  relative  to 
the  ivory  scale,  that  is,  in  a  plane  perpendicular  to  the  scale  and 
passing  through  the  center  graduation.  A  mark  may  be  made  on 
the  glass  door  by  which  to  line  up  the  eye  before  each  reading. 
The  final  reading  must  be  made  with  the  door  closed. 

Arrest  the  swinging  of  the  balance  when  the  pointer  is  at  the 
center  of  the  scale.  This  prevents  any  undue  jarring  of  the  beam, 
which  is  very  likely  to  get  the  balance  out  of  adjustment.  Always 
turn  the  balance  out  of  action  before  adding  weights  to  the  pan 


78  A   TEXTBOOK  OF  FIRE  ASSAYING 

or  taking  them  from  it.  When  the  balance  is  not  in  use,  raise 
the  beam  from  the  knife-edges  and  leave  the  rider  on  the  beam. 

Do  not  allow  the  direct  rays  of  the  sun  to  strike  the  balance 
and  never  attempt  to  do  close  weighing  unless  the  temperature 
of  the  room  and  balance  can  be  maintained  virtually  constant. 

Each  silver  bead  should  be  placed  on  its  side  on  a  small  anvil, 
hammered  and  then  brushed  before  it  is  weighed. 

To  transfer  the  gold  from  the  parting  cup  to  the  scale-pan, 
take  the  scale-pan  with  the  forceps  and  place  on  the  front  part 
of  the  glass  mounting  base.  Gradually  invert  the  parting  cup 
over  it,  tapping  it  gently.  The  gold  should  all  slide  into  the  pan. 
Any  particles  adhering  to  the  cup  may  be  detached  by  touching 
gently  with  the  point  of  the  forceps  or  by  means  of  a  small  feather 
trimmed  to  a  point. 

Before  weighing  the  gold,  examine  it  carefully  to  see  if  it  is 
clean  and  remove  any  foreign  matter  if  present. 

To  remove  gold  from  the  scale-pan  after  weighing,  pick  up  pan 
and  all  in  the  forceps  and  invert  pan  over  the  parting  cup,  brush- 
ing off  lightly  at  the  same  time. 

Weights  should  be  placed  only  in  the  box  or  on  the  scale-pan  and 
should  be  handled  only  with  ivory-tipped  forceps.  Record  the 
weight  of  the  substance  first,  by  noting  the  weights  which  are 
absent  from  the  box,  second,  by  checking  off  each  weight  as  it  is 
put  back  in  the  box.  Record  all  weights  in  the  notebook  and  not 
on  scraps  of  paper. 

For  ordinarily  accurate  commercial  work  the  weighing  of  the 
gold  and  silver  is  done  by  the  "  method  of  equal  swings,"  using 
the  rider  for  the  final  weighing.  For  extreme  accuracy,  as  for 
instance  in  the  calibration  of  weights,  the  weighing  is  done  by 
"  deflection,"  also  called  the  "  method  of  swings." 

Weighing  by  "  Equal  Swings."  —  First  of  all,  the  balance  is 
adjusted  by  the  star  wheel  or  preferably  by  the  adjusting  rider,  if 
one  is  provided,  until  the  needle  swings  exactly  the  same  distance 
on  each  side  of  the  center,  reading  always  in  the  same  order,  say 
from  left  to  right.  For  accurate  gold  weighing  it  will  be  necessary 
to  estimate  tenths  of  divisions  on  the  ivory  scale. 

Put  the  substance  to  be  weighed  on  the  left-hand  pan  and  add 
weights  to  the  right-hand  pan  until  their  weight  is  within  a  frac- 
tion of  a  milligram  of  the  weight  of  the  substance.  Apply  the 
weights  in  a  systematic  manner,  starting  with  one  which  is  esti- 


BALANCES  AND  WEIGHTS  79 

mated  to  be  too  large.  If  too  large,  remove  it  and  try  the  next 
smaller  weight  always  working  from  larger  to  smaller  weights 
until  within  1  milligram  of  the  true  weight. 

In  trying  any  weight  have  the  beam  off  the  knife-edges,  put  the 
weight  in  the  pan  and  gently  turn  the  balance  key  until  the 
pointer  inclines  slightly  to  one  side  or  the  other.  This  swing  of 
only  one  or  two  divisions  should  indicate  immediately  whether 
the  weight  on  the  pan  is  too  much  or  too  little.  Again  turn  the 
balance  out  of  action  before  making  any  change  of  weight. 

When  within  a  fraction  of  a  milligram  of  the  correct  weight, 
shift  the  right-hand,  or  weighing,  rider  about,  until,  when  the 
balance  is  put  into  action  the  needle  does  not  move  very  decidedly 
in  one  or  the  other  direction.  Then  set  the  beam  swinging  1 
or  2  divisions  each  side  of  the  center.  If  it  does  not  swing  evenly 
arrest  the  swing,  change  the  position  of  the  rider  and  try  again. 
Repeat  until  the  needle  swings  exactly  as  when  adjusted.  After 
one  has  become  familiar  with  the  balance  only  two  or  three  trials 
of  the  rider  will  be  necessary. 

The  weight  of  the  substance  is  found  from  the  sum  of  the  weights 
on  the  pan  plus  the  fractional  part  of  a  milligram  indicated  by 
the  position  of  the  rider  on  the  beam. 

Weighing  by  "  Method  of  Swings."  —  First,  determine  the 
point  of  rest  under  zero  load  by  noting  the  position  of  the  pointer 
at  the  extreme  swing  on  each  side,  taking  3,  5  or  a  greater  odd 
number  of  consecutive  readings.  Call  the  center  division  zero 
and  count  divisions  and  estimate  tenths  to  each  side,  calling  those 
to  the  left  of  the  center  —  ,  and  to  the  right  +. 

Average  the  readings  for  each  extreme,  add  the  two  and  divide 
the  sum  by  2;  the  result  is  the  point  of  rest.  The  method  is 
illustrated  in  the  following  example. 

Left  Right 

-3.9  3.6 

-3.7  3.4 

-3.5  2(7.0 

3.5 


point  of  rest. 


80      '  A   TEXTBOOK  OF  FIRE  ASSAYING 

Or  the  point  of  rest  would  be  0.1  division  to  the  left  of  the 
center. 

Call  the  point  of  rest  under  zero  load  r.  Place  the  object  to  be 
weighed  on  the  left-hand  pan  and  weights  on  the  right-hand  pan 
until  equilibrium  is  nearly  established.  With  the  rider  determine 
the  weight  to  the  next  smaller  0.1  milligram.  Set  the  beam  swing- 
ing as  before  and  find  the  position  of  rest  for  the  pointer.  Call  it 
r!  Shift  the  rider  to  the  right,  one  whole  division  (=0.1  mg.) 
so  as  to  bring  the  point  of  rest  on  the  opposite  side  of  r,  find  the 
position  of  rest  again,  and  call  it  r" .  The  fraction  of  a  milli- 
gram to  be  added  to  the  weights  and  rider  reading  when  r'  was 
found  is  then 

r!  ~  r,,  x  o.io 


For  instance  let  the  weights  and  rider  reading  be  27.4  mg.  and 
let  r'  =  -  1.4  and  r"  =  +  1.6 

.,       r'  -  r          -  1.4  +  0.1        -  1.3 

then  p—p  .     rT4=Te  =:  ^o  =    +  0-43 

and  the  true  weight  would  be  27.4  +  (0.43  X  0.1)  =  27.44  mg. 

Another  method  of  weighing  by  "  deflection,"  requiring  a 
knowledge  of  the  sensitivity  of  the  balance,  is  as  follows:  Sup- 
pose that  a  weight  of  0.10  milligram  will  cause  a  deflection  of  the 
point  of  rest  of  2.0  divisions  on  the  ivory  scale.  Adjust  the  bal- 
ance so  that  the  point  of  rest  with  no  load  corresponds  to  the 
zero  of  the  ivory  scale.  Place  the  substance  to  be  weighed  in  the 
left-hand  pan  and  again  determine  the  point  of  rest.  Suppose 
the  deflection  to  be  1.2  divisions.  Then  the  weight  of  the  sub- 
stance is  0.06  milligram.  With  a  good  balance  this  is  a  rapid  and 
accurate  method  for  small  amounts  of  gold,  but  it  is  not  very 
commonly  used. 

Weighing  by  "  No  Deflection."  —  A  third  method  of  weighing, 
called  weighing  by  "  no  deflection,"  is  sometimes  employed  for 
rough  work.  It  consists  in  applying  the  necessary  weights  and 
then  shifting  the  rider  until  the  needle  shows  no  deflection  when 
the  balance  is  lowered  gently  onto  the  knife-edges.  This  method 
disregards  friction  and  inertia  and  is  not  as  accurate  as  the  two 
previously  described  methods. 

Weighing  by  Substitution.  —  This  method  of  weighing  is  the 
one  usually  adopted  for  the  standardization  or  adjustment  of 


BALANCES  AND   WEIGHTS  81 

weights,  as  it  avoids  any  possibility  of  error  due  to  inequality  of 
arms.  It  consists  simply  in  placing  the  substance  to  be  weighed 
on  one  pan,  counterbalancing  it  with  weights  placed  on  the  other 
pan,  and  then  removing  the  substance  and  adding  standard  weights 
until  the  balance  is  again  in  equilibrium.  The  weight  of  the  sub- 
stance is  obtained  from  the  substituted  weights. 

Check  Weighing.  —  Students  are  advised  to  check  all  gold 
weighings  in  the  following  manner:  Weigh  and  record  weight 
of  each  lot  of  parted  gold  resulting  from  duplicate  or  triplicate 
assays,  then  place  all  on  one  scale-pan  and  obtain  the  total  weight. 
Compare  this  with  the  sum  of  the  weights  obtained  in  the  separate 
weighings.  The  figures  should  check  within  0.01  or  at  most 
0.02  milligram.  If  they  do  not,  some  of  the  weighings  are  at 
fault,  some  of  the  weights  are  in  error,  or  the  zero  point  has  changed. 
By  weighing  the  combined  gold  from  2  or  3  assays  and  reducing 
to  milligrams  per  assay-ton,  the  accuracy  of  the  assay  is  corre- 
spondingly increased.  This  practice  is  followed  by  all  good 
assay  ers. 

Accumulative  Weighing.  —  A  modification  of  the  above  method 
of  check  weighing  is  to  weigh  the  gold  accumulatively.  For  in- 
stance, suppose  an  assay er  has  fifty  lots  of  gold  to  weigh,  each  one 
of  them  perhaps  less  than  1  milligram.  He  can  save  time  by 
weighing  one  after  the  other,  without  bothering  to  remove  the 
previous  lot,  until  all  fifty  are  on  the  scale-pan  at  one  time. 
He  records,  of  course,  after  the  first  weighing,  the  difference  of 
weight  caused  by  each  increment  of  gold.  Besides  saving  time, 
this  method  of  weighing  reduces  to  a  negligible  amount  any  con- 
stant error,  such  as  change  of  adjustment,  as  instead  of  occurring 
in  each  one  of  the  fifty  weighings,  the  full  amount  of  this  error 
occurs  only  once  in  all,  and  but  one-fiftieth  of  it  applies  to  any  one 
weighing. 

ADJUSTING  AND  TESTING  AN  ASSAY  BALANCE. 

Leveling.  —  Level  the  balance  by  adjusting  the  footscrews  and 
by  observing  the  plumb-bob  or  level.  Be  sure  that  it  rests  firmly 
on  the  table  or  other  support  so  that  it  will  not  be  moved  during 
the  test.  See  that  the  beam,  scale-pans  and  hangers  are  in  their 
proper  places  and  have  not  been  forced  out  of  normal  position  by 
previous  careless  usage. 


82  A   TEXTBOOK  OF  FIRE  ASSAYING 

Equilibrium.  —  Lower  the  beam  carefully  until  the  agate  knife- 
edges  rest  on  the  agate  supports.  This  motion  and  the  reverse 
one  must  be  gentle  to  prevent  injury  to  the  knife-edges  and  also 
to  prevent  a  shock  or  jar,  which  would  tend  to  change  the  adjust- 
ments. Adjust  the  balance  so  that  the  pointer  swings  equally 
on  each  side  of  the  center.  A  star  wheel,  a  small  projecting  piece 
of  metal  or  "  flag,"  revolving  on  a  vertical  axis  at  the  middle  of 
the  beam,  or  preferably  an  extra  rider,  constitutes  the  attachment 
for  this  adjustment.  If  this  adjustment  cannot  be  made  and 
the  balance  on  starting  to  one  side  or  the  other  continues  to  swing 
in  that  direction  with  increasing  velocity,  it  is  in  unstable  equilib- 
rium, and  the  center  of  gravity  must  be  lowered  until  the  proper 
equilibrium  is  obtained. 

Time  of  Oscillation.  —  Set  the  balance  in  motion  and  note  the 
time  of  one  complete  oscillation,  i.e.,  swing  from  one  extreme 
to  the  other  and  back  again.  For  the  modern  5-inch-beam  assay 
balance  this  should  be  from  twelve  to  fifteen  seconds.  If  much 
faster  than  this  the  balance  will  probably  not  be  very  sensitive. 
If  much  slower  than  this  the  balance  may  lack  stability  and  each 
weighing  will  take  a  correspondingly  longer  time. 

Lowering  the  center  of  gravity  of  the  beam  results  in  decreasing 
the  time  of  oscillation. 

Stability.  —  By  "  stability  "  of  a  balance  is  meant  its  property 
of  remaining  in  adjustment  during  use  and  in  spite  of  moderate 
changes  of  room  temperature.  It  is  a  common  error  of  assayers 
to  neglect  testing  for  stability  when  selecting  a  fine  balance,  and 
yet  stability  is  fully  as  important  as  a  high  degree  of  sensitiveness. 

After  each  of  the  tests  the  beam  should  be  lowered  and  the 
adjustment  of  the  balance  noted.  If  it  no  longer  swings  equally 
on  each  side  of  the  center,  due  care  having  been  taken  to  avoid 
disturbing  any  of  the  settings,  it  lacks  stability.  This  may  be  due 
to  excessive  sensitiveness,  which  can  be  overcome  by  lowering  the 
center  of  gravity  of  the  system  by  means  of  the  screw-ball,  or  it 
may  be  due  to  a  defect  in  construction,  arms  of  unequal  length, 
etc.,  in  which  case  it  cannot  be  remedied. 

Resistance.  —  If  the  knife-edges  are  dull  or  the  supporting 
surfaces  rough  the  frictional  resistance  to  swinging  will  be  con- 
siderable and  the  diminution  in  the  amplitude  of  swing  will  be 
rapid.  Note  the  position  of  the  pointer  on  the  scale  at  the  ex- 
tremes of  several  successive  swings.  The  difference  between 


BALANCES  AND  WEIGHTS  83 

successive  readings  on  the  same  side  will  show  the  diminution  in 
amplitude  due  to  friction  and  to  resistance  of  the  air.  In  a 
good  assay  balance  this  should  not  exceed  0. 1  of  a  division  when 
the  amplitude  of  swing  is  1  division.  The  horizontal  section  of 
the  beam  and  the  area  of  the  pans  and  other  projecting  parts 
should  be  as  small  as  possible,  to  reduce  the  air  resistance. 

Let  the  balance  swing  until  it  comes  to  rest  and  read  the  po- 
sition of  the  pointer,  lift  the  beam  from  the  knife-edges  and  repeat 
several  times.  The  positions  should  not  differ  by  more  than  0.05 
of  a  division.  A  greater  difference  than  this  indicates  flatness 
of  the  knife-edges  or  roughness  of  the  supporting  surfaces.  If 
the  beam  is  exceedingly  slow  in  coming  to  rest  this  test  is  unneces- 
sary. 

Sensitivity.  —  The  sensitiveness  of  a  balance  is  defined  by 
physicists  as  the  angle  through  which  the  beam  moves  when  1 
milligram  excess  weight  is  added  to  one  pan.  If  the  scale  gradu- 
ations are  laid  out  on  the  arc  of  a  circle  whose  radius  is  the  dis- 
tance from  the  center  knife-edge  to  the  scale,  the  number  of  scale 
divisions  passed  over  are  proportional  to  the  angle  of  deflection 
and  in  any  given  balance,  may  be  taken  as  a  measure  of  the  sen- 
sitiveness. Unfortunately,  however,  there  is  as  yet  no  stand- 
ard distance  between  scale  graduations  and  no  uniformity  of 
length  of  pointer,  so  that  the  number  of  scale  divisions  passed 
over  cannot  be  used  directly  as  a  means  of  comparing  the  sen- 
sitiveness of  balances  of  different  makes. 

From  a  practical  point  of  view,  the  sensitiveness  is  the  smallest 
difference  in  weight  which  the  balance  will  indicate.  Thus,  when 
we  say  that  a  balance  is  sensitive  to  0.01  milligram  we  mean  that 
0.01  milligram  added  to  one  pan  will  cause  a  noticeable  difference 
in  the  swing  or  in  the  position  of  the  point  of  rest. 

Comparative  Sensitivity.  —  With  a  distance  between  scale 
graduations  of  0.05  inches  it  is  easily  possible  to  estimate  the  po- 
sition of  the  pointer  at  each  extreme  of  a  swing  to  the  nearest 
0.2  division  or  to  within  0.01  inch.  Pointers  on  a  number  of  the 
better  American  assay  balances  range  from  5.5  to  6.75  inches  in 
length  and  average  about  6  inches.  With  the  usual  length  of 
pointer,  the  position  of  the  point  of  rest  should  be  shifted  at  least 
0.01  inch  when  an  unbalanced  weight  of  0.01  milligram  is  placed 
in  one  pan,  if  the  balance  is  to  be  termed  sensitive  to  0.01  milli- 
gram. With  this  as  a  basis  anyone  may  work  out  his  own  method 


84  A   TEXTBOOK  OF  FIRE  ASSAYING 

of  comparing  the  sensitiveness  of  different  balances,  by  taking  into 
account  the  distance  between  graduations  and  the  length  of  the 
pointer. 

To  Test  Equality  of  Anns.  —  Adjust  balance  to  swing  evenly 
with  no  load  and  then  place  equal  weights  on  each  pan,  equivalent 
to  the  full  load  of  the  balance.  If  the  pointer  does  not  now  swing 
evenly  the  arms  are  of  unequal  length. 

To  Determine  if  Knife-Edges  are  all  in  Same  Horizontal  Plane. 
—  Adjust  balance  and  determine  sensitivity  with  no  load.  Then 
place  full  load  on  each  pan  and  again  determine  sensitivity.  When 
the  three  knife-edges  are  in  the  same  plane  there  should  be  no 
change  of  sensitivity  with  any  weight  up  to  the  full  load  of  the 
balance.  When  the  full  load  of  the  balance  is  not  known  the 
sensitivity  should  be  determined  for  gradually  increasing  loads  and 
a  curve  of  sensitivity  drawn.  If  the  three  knife-edges  are  in  the 
same  plane  this  curve  should  be  a  straight  line  up  to  the  point 
where  the  beam  begins  to  be  deflected  by  an  overload. 

WEIGHTS. 

For  the  three  balances  above  described  we  require  four  sets  of 
weights,  as  follows: 

For  the  flux  balance  we  should  have  a  block  containing  weights 
from  1  kilogram  to  1  gram.  These  weights  need  not  be  extremely 
accurate. 

For  the  pulp  balance  two  sets  are  necessary,  gram  and  assay-ton 
weights :  gram  weights,  from  20  grams  to  10  milligrams  for  weigh- 
ing flour  and  ore  for  lead,  copper  and  tin  assays,  as  well  as  the 
buttons  from  the  same :  assay-ton  weights,  2  A.  T.  to  y^  A.  T.  for 
weighing  ore,  matte,  speiss  and  lead  bullion  for  the  gold  and  silver 
assay. 

For  the  button  balance  is  required  a  set  of  milligram  weights  of 
the  utmost  accuracy,  from  1  milligram  up  to  500  or  1000  milli- 
grams. These  are  preferably  made  of  platinum,  as  an  absolutely 
non-corrosive  weight  is  imperative.  Riders  are  used  for  deter- 
mining fractions  of  a  milligram.  Riders  are  made  of  fine  aluminum 
wire  and  are  usually  made  to  weigh  0.50  or  1.00  milligram.  The 
balance  beam  is  usually  divided  into  100  spaces  on  each  side  of 
the  center  and  when  a  1-milligram  rider  is  used  each  space  repre- 
sents 0.01  milligram. 

For  many  balances,  a  rider  with  a  diamond-shaped  loop,  known 


BALANCES  AND  WEIGHTS  85 

as  the  Thompson  rider,  is  to  be  preferred.  Its  principal  advan- 
tage is  due  to  its  property  of  always  hanging  in  a  vertical  position 
when  on  the  rider  arm.  Even  if  it  falls  over  to  one  side  when  on 
the  beam  it  will  slip  back  to  the  vertical  position  when  lifted  by 
the  rider  arm.  The  diamond-shaped  loop  prevents  it  from  swing- 
ing or  twisting  around  on  the  rider  carrier  and  permits  the  rider 
to  be  placed  squarely  on  the  beam. 


FIG.  38.  —  Thompson  multiple  rider  attachment. 

Multiple  Rider  Attachment.  —  Some  of  the  balance  makers  are 
now  supplying,  on  demand,  what  is  called  a  multiple  rider  attach- 
ment, designed  to  do  away  with  the  use  of  the  smaller  weights. 
It  consists  of  a  carrier  supplied  with  a  number  of  riders  of  different 
weights,  for  instance,  1,  2,  3,  5,  10,  20,  30  milligrams,  so  arranged 
that  any  or  all  may  be  placed  on  a  support  provided  for  the  pur- 
pose. This  is  equivalent  to  placing  flat  weights  of  the  same 
value  in  the  pan. 

In  Fig.  38  is  shown  a  multiple  rider  attachment,  the  horizontal 
arm  of  which  extends  through  the  glass  side  of  the  balance  and 


86  A   TEXTBOOK  OF  FIRE  ASSAYING 

terminates  in  a  milled  head.  The  different  riders  are  distinguished 
from  each  other  by  differently  formed  ends.  An  advantage 
claimed  for  this  device  is  a  saving  in  the  wear  and  tear  of  weights, 
as  the  small  flat  weights,  frequently  handled  by  forceps,  become 
broken  and  inaccurate,  whereas  the  riders,  on  which  there  is  prac- 
tically no  wear,  will  maintain  their  original  weight  almost  indefi- 
nitely. A  second  advantage  claimed  is  a  saving  in  time,  as  with 
this  attachment  the  riders  can  be  manipulated  much  more  quickly 
than  the  flat  weights,  which  must  be  handled  with  forceps.  It  is 
not  necessary  to  open  the  door  of  the  balance  in  weighing  any  bead 
under  40  or  50  milligrams,  and  this  alone  is  a  saving  of  some  time 
and  also  allows  all  air  currents  to  subside  before  the  final  reading  is 
made. 

Assay-Ton  Weights.  —  The  assay-ton  system  of  weights  was 
devised  to  facilitate  the  calculation  of  the  results  of  gold  and  silver 
assays.  In  the  United  States  and  Canada  the  results  of  such  as- 
says are  reported  in  troy  ounces  of  gold  and  silver  per  2000  pound 
avoirdupois  ton  of  ore.  With  the  ordinary  system  of  weights  a 
tedious  calculation  would  have  to  be  made  for  each  assay  with 
the  possibility  of  mathematical  errors. 

The  basis  of  the  assay-ton  system  is  the  number  of  troy  ounces 
(29,166.+)  in  one  ton  of  2000  pounds  avoirdupois.  The  assay- 
ton  is  made  to  weigh  29.166  grams.  Then 

1  ton  avoirdupois  :  1  ounce  troy  :  :  1  assay-ton  :  1  milligram. 
Therefore,  with  one  assay-ton  of  ore,  the  weight  of  the  silver  or 
gold  in  milligrams  gives  immediately  the  assay  in  ounces  per  ton. 

In  England  and  Australia  the  long  ton  of  2240  pounds  is  used, 
and  the  assay-ton  weighs  32.666  grams. 

In  Mexico  ores  are  bought  and  sold  in  metric  tons  of  1000 
kilograms  and  assays  are  reported  in  grams  of  gold  and  grams  or 
kilograms  of  silver  per  metric  ton.  In  this  case  it  is  convenient 
to  weigh  out  ore  in  grams. 

Calibration  of  Weights.  —  The  weights  supplied  by  the  makers 
cannot  always  be  relied  upon  and  even  originally  perfect  ones  are 
subject  to  changes  of  weight  due  to  wear  or  accumulation  of  dirt; 
Therefore  ;t  behooves  the  assayer  to  check  his  weights  Occasionally 
and  to  determine  the  correction  to  be  applied  to  the  marked  value. 
This  requires  the  use  of  a  standardized  weight  which  should  be 
carefully  preserved  and  used  for  this  purpose  only. 

The  method  of  swings  should  be  used  and  the  weighing  done  by 


BALANCES  AND  WEIGHTS  87 

deflection  after  the  sensitivity  of  the  balance  has  been  determined. 
First  determine  the  position  of  rest  and  the  sensitivity  with  no 
load,  with  100,  250  and  500  milligram  loads  respectively.  The 
sensitivity  should  not  vary  much  throughout  this  range.  The 
method  to  be  followed  can  be  understood  from  the  following 
example. 

CALIBRATION  OF  A  SET  OF  ASSAY  WEIGHTS. 
Designate  each  weight  by  its  marked  value  in  the  parenthesis 
and  when  there  are  several  of  the  same  value  note  some  peculiarity 
by  which  each  may  be  designated.     The  weights  in  the  set,  marked 
in  milligrams  are : 

(500)  =  a,  (200)  =  6,  (100)  =  c,  (1000  =  d,  (50)  =  e,  (20)  =  /, 
(10)  =  g,  (100  =  h,  (10")  =  (5)  +  (2)+  (20  +  (1)  =  i. 

The  weight  (100)  is  compared  with  the  standard  100-milligram 
weight  and  the  weights  are  then  compared  among  themselves  by 
the  method  of  swings.     The  letters  represent  the  true  values. 
In  calibrating  the  weights  from  100  milligrams  to  10  milligrams, 
observations  should  be  made  on  the  following  combinations: 
Left-hand  Pan  Right-hand  Pan 

(100)  100  mg.  standard  ,  s  ' 

(100)  (50)  +  (20)  +  (10)  +  (100  +  (5)  +  (2)  +  .(2'|) 

+  (D 

(50)  (20)  +  (10)  +  (100  +  (5)  +  (2)  +  (20  +  (1) 

(20)  (10)  +  (100 

The  recorded  observations  are  as  follows: 

100  mg.  =  c  -  0.020mg. 


C               =6 

+   / 

+   </   +  /*   + 

;  + 

0. 

190 

e               =  f 

+ 

•  o 

+  h  +  i 

+ 

0. 

020 

f           =9 

-1 

-\ 

+ 

0. 

040 

g         =h 

+ 

0. 

015 

h         =i 

+ 

0. 

040 

Solving  these  equations, 

i 

— 

i 

h 

= 

i  +  0.040 

g 

= 

i  +  0.055 

/ 

= 

2i  +  0.135 

e 

= 

5i  +  0.250 

c 

= 

10*'  +  0.670 

' 

c 

•• 

100.020  mg. 

88  A    TEXTBOOK  OF  FIRE  ASSAYING 

From  the  last  two  values  of 

lOi  =  99 . 350  mg. 
i  =    9.935mg. 

Substituting  this  value  for  i  in  the  above  equations,  we  find  the 
following  values  for  the  other  weights  of  the  set: 

Designation       Actual  Weight      Correction*  to  Marked  Value 

c  100.020  +  .020mg. 

e  49.925  -  .075   " 

/  20.005  +  .005    " 

g  9.990  -  .010  " 

h  9.975  -  .025   " 

i  9.935  -  .065   " 

The  smaller  weights  in  i,  may  be  calibrated  in  a  similar  manner. 
The  large  weights  (100),  (200)  and  (500)  may  be  standardized  by  a 
simple  modification  of  the  above. 

The  process  is  made  much  simpler  by  having  a  complete  set  of 
standard  weights  which  are  very  carefully  handled  and  kept 
solely  for  standardizing  purposes,  and  these  the  larger  assay  offices 
usually  have. 

Testing  Riders.  —  Every  new  rider  should  be  tested  before  use, 
as  riders  often  vary  0.01  or  0.02  milligrams  from  their  supposed 
value.  If  a  rider  is  too  heavy,  a  little  bit  at  a  time  may  be  cut  off 
with  a  pair  of  scissors  until  it  comes  down  to  the  standard. 

*  A  +  correction  means  that  the  weight  is  heavier  than  the  normal  value. 


CHAPTER  V. 
CUPELLATION. 

In  every  assay  of  an  ore  for  gold  and  silver,  we  endeavor  to  use 
such  fluxes  and  to  have  such  conditions  as  will  give  us  as  a  result- 
ant two  products: 

1st.  An  alloy  of  lead,  with  practically  all  of  the  gold  and  silver 
of  the  ore  and  as  small  amounts  of  other  elements  as  possible. 

2nd.  A  readily  fusible  slag  containing  the  balance  of  the  ore 
and  fluxes. 

The  lead  button  is  separated  from  the  slag  and  then  treated  by 
a  process  called  cupellation  to  separate  the  gold  and  silver  from 
the  lead.  This  consists  of  an  oxidizing  fusion  in  a  porous  vessel 
called  a  cupel.  If  the  proper  temperature  is  maintained  the  lead 
oxidizes  rapidly  to  PbO  which  is  partly  (98.5  *  per  cent) 
absorbed  by  the  cupel  and  partly  (1.5  per  cent)  volatilized. 
When  this  process  has  been  carried  to  completion  the  gold  and  sil- 
ver is  left  in  the  cupel  in  the  form  of  a  bead. 

The  cupel  is  a  shallow,  porous  dish  made  of  bone-ash,  Portland 
cement,  magnesia  or  other  refractory  and  non-corrosive  material. 
The  early  assayers  used  cupels  of  wood  ashes  from  which  the 
soluble  constituents  had  been  leached.  Agricola,  writing  about 
the  year  1550,  mentions  the  use  of  ashes  from  burned  bones. 
Ashes  from  deers'  horns  he  pronounces  best  of  all;  but  the  use 
of  these  was  becoming  obsolete  in  his  time,  and  he  states  that 
assayers  of  his  day  generally  made  the  cupels  from  the  ashes  of 
beech  wood. 

To-day  it  is  thought  that  the  bones  of  sheep  are  the  best  for 
cupels.  The  bones  should  be  cleaned  before  burning  and  as 
little  silica  as  possible  introduced  with  them.  It  is  important 
not  to  burn  the  bones  at  too  high  a  temperature  as  this  makes 
the  ash  harder  and  less  absorbent.  It  is  also  advisable  to  boil 
the  bones  in  water  before  burning  them  as  this  dissolves  a  great 
part  of  the  organic  matter,  which  if  burned  with  the  bones  yields 
sulphates  and  carbonates  of  the  alkalies. 

*  Liddell,  Eng.  and  Min.  Jour.  89,  p.  1264.     (June,  1910.) 

89 


90 


A   TEXTBOOK  OF  FIRE  ASSAYING 


Properly  burned  sheep  bones  will  yield  an  ash  containing  about 
90  per  cent  calcium  phosphate,  5.65  per  cent  calcium  oxide, 
1.0  per  cent  magnesium  oxide,  and  3.1  per  cent  calcium  fluoride. 
Ordinary  commercial  bone-ash  also  contains  more  or  less  silica 
and  unoxidized  carbon.  If  more  than  a  fraction  of  a  per  cent  of 
silica  is  found  in  bone-ash,  it  is  evidence  that  sufficient  care  has 
not  been  taken  in  cleaning  the  bones,  and  cupels  made  from  such 
bone-ash  are  more  likely  than  others  to  crack  during  cupellation, 
often  resulting  in  the  loss  of  small  beads.  If  the  bone-ash  shows 
black  specks  it  is  an  indication  of  insufficient  oxidation  and  the 
assayer  should  allow  the  cupels  to  stand  for  some  time  in  the 
hot  muffle,  with  the  door  open,  before  using.  Carbon  is  an 
undesirable  constituent  of  cupels  as  it  reacts  with  the  lead  oxide 
formed  giving  off  CO  and  CO2  which  may  cause  a  loss  of  the 
molten  alloy  due  to  spitting. 

Bone-ash  for  cupels  should  be  finely  ground  to  pass  at  least  a 
40-mesh  screen  and  the  pulverized  material  should  consist  of 
such  a  natural  mixture  of  sizes  as  will  give  a  solid  cupel  with 
enough  fine  material  to  fill  interstices  between  coarser  particles. 
Opinions  differ  as  to  the  best  size  of  crushing  for  bone-ash 
and  this  will  depend  no  doubt  upon  the  character  of  the  material. 
The  bone-ash  represented  by  the  following  screen  analysis  has, 
however,  yielded  particularly  good  cupels. 

TABLE  VIII. 
SIZE  OF  BONE-ASH. 


Size  mesh 

Size  mm. 

Per  cent  weight 

On    40 

0.380 

9 

Through  40   "     60 

0.244 

14 

60   "   100 

0.145 

17 

100   "    150 

0.098 

10 

150 

50 

With  cupels  made  from  this  bone-ash  it  was  possible  to  reduce 
losses  to  1.60  per  cent,  using  100  mg.  of  silver  and  25  grams  of 
lead,  while  with  some  other  lots  of  bone-ash  containing  smaller 
proportions  of  —  150-mesh  material  it  was  found  impossible  to 
keep  the  losses  below  2.0  per  cent. 


CUPELLATION  91 

Making  Cupels.  —  Cupels  are  made  by  moistening  the  bone- 
ash  with  from  8  to  20  per  cent  of  water  and  compressing  in  a  mold. 
The  bone-ash  and  water  should  be  thoroughly  mixed  by  knead- 
ing, and  the  mixture  should  finally  be  sifted  through  a  10-  or 
12-mesh  sieve  to  break  up  the  lumps.  Some  authorities  recom- 
mend adding  a  little  potassium  carbonate,  molasses  or  flour  to 
the  mixture,  but  with  good  bone-ash  nothing  but  pure  water 
need  be  added.  The  mixture  should  be  sufficiently  moist  to  co- 
here when  strongly  squeezed  in  the  hands,  but  not  so  wet  as  to 
adhere  to  the  fingers  or  to  the  cupel  mold.  Twelve  per  cent  of 
water  by  weight  is  about  right;  but  the  amount  used  will  depend 


FIG.  39.  —  Cupel  machine. 

somewhat  on  the  bone-ash  and  on  the  pressure  used  in  forming 
the  cupels.  The  greater  the  pressure  the  smaller  the  amount  of 
water  which  is  required.  It  is  better  to  err  on  the  side  of  making 
the  mixture  a  little  too  dry  than  too  wet. 

The  cupels  may  be  molded  either  by  hand  or  by  machine.  The 
hand  outfit  consists  of  a  ring  and  a  die.  The  ring  is  placed  on 
the  anvil  and  filled  with  the  moist  bone-ash,  the  die  is  inserted 
and  pressed  down  firmly.  It  is  then  struck  one  or  more  blows 
with  a  heavy  hammer  or  mallet,  and  turned  after  each  blow; 
finally  the  cupel  is  ejected.  The  cupels  are  placed  on  a  board 
and  dried  slowly  in  a  warm  place.  The  amount  of  compression 
is  a  matter  of  experience  and  no  exact  rule  for  it  can  be  given; 


92  A   TEXTBOOK  OF  FIRE  ASSAYING 

but  it  may  be  approximated  by  making  the  cupels  so  hard  that 
when  removed  from  the  mold  they  are  scratched  only  with  diffi- 
culty by  the  finger  nail.  One  man  can  make  about  100  cupels 
an  hour,  using  the  hand  mold  and  die. 

Several  types  of  cupel  machines  are  on  the  market  and  one  of 
the  best  is  shown  in  Fig.  39.  This  machine,  has  a  compound 
lever  arrangement  which  gives  a  pressure  on  the  cupel  equal  to 
twenty  times  that  applied  to  the  hand  lever.  By  adjusting, 
different  degrees  of  compression  may  be  obtained.  These 
machines  have  interchangeable  dies  and  rings  so  that  different 
sizes  of  cupels  may  be  made.  The  rated  capacity  of  this  machine 
is  two  hundred  cupels  an  hour.  Another  machine  made  by  the 
same  company  has  an  automatic  charging  arrangement.  This 
machine  is  claimed  to  have  a  capacity  of  six  hundred  cupels  an 
hour.  Cupels  should  be  uniform  in  hardness,  and  it  would  seem 
that  with  a  properly  designed  machine  a  more  uniform  pressure 
could  be  obtained  than  by  the  use  of  hammer  and  die.  Some 
assayers,  however,  still  prefer  hand-made  cupels. 

Cupels  should  be  air-dried  for  several  days,  at  least,  before 
use.  Most  assayers  make  them  up  several  months  in  advance 
so  as  to  insure  complete  drying.  They  should  not  be  kept  where 
fumes  from  parting  can  be  absorbed  by  them  as,  if  this  occurs, 
the  CaO  present  will  be  converted  into  Ca(NO3)2.  This  com- 
pound is  decomposed  at  the  temperature  of  cupellation  and  may 
cause  spitting  of  the  lead  button. 

Cupels  should  not  crack  when  heated  in  the  muffle  and  should 
be  so  strong  that  they  will  not  break  when  handled  with  the 
tongs.  Good  cupels  give,  a  slight  metallic  ring  when  struck  to- 
gether after  air-drying.  It  is  best  to  heat  cupels  slowly  in  the 
muffle  as  this  lessens  the  chance  of  their  cracking. 

A  good  cupel  should  be  perfectly  smooth  on  the  inside,  and  of 
the  right  porosity.  If  it  is  too  dense,  the  time  of  cupellation  is 
prolonged  and  the  temperature  of  cupellation  has  to  be  higher, 
thus  increasing, the  loss  of  silver.  If  the  cupel  is  too  porous  it 
is  said  that  there  is  danger  of  a  greater  loss,  due  to  the  ease  with 
which  small  particles  of  alloy  can  pass  into  the  cupel.  The 
bowl  of  the  cupel  should  be  made  to  hold  a  weight  of  lead  equal 
to  the  weight  of  the  cupel. 

The  shape  of  the  cupel  seems  to  influence  the  loss  of  precious 
metals.  A  flat,  shallow  one  exposes  a  greater  surface  to  oxida- 


CUPELLATION  93 

tion  and  allows  of  faster  cupellation;  it  also  gives  a  greater  sur- 
face of  contact  between  alloy  and  cupel,  and  as  far  as  losses  are 
due  to  direct  absorption  of  alloy,  it  will  of  course  increase  these. 
The  writer,  using  the  same  bone-ash  and  cupel  machine,  and 
changing  only  the  shape  of  the  cupel,  has  found  shallow  cupels  to 
give  a  much  higher  loss  of  silver.  In  doing  this  work  it  was 
found  harder  to  obtain  crystals  of  litharge  with  the  shallow  cupel 
without  freezing,  and  it  was  very  evident  that  a  higher  cupella- 
tion temperature  was  required  for  the  shallow  cupel.  The  reason 
for  this  is  that  in  the  case  of  the  shallow  cupel  the  molten  alloy 
is  more  directly  exposed  to  the  current  of  air  passing  through 
the  muffle,  and  consequently  a  higher  muffle  temperature  has 
to  be  maintained  to  prevent  freezing.  T.  K.  Rose*  also  prefers 
deep  cupels  on  account  of  smaller  losses.  French  found  shallow 
cupels  less  satisfactory  on  account  of  sprouting. 

A  satisfactory  size  of  cupel  for  general  assay  work  is  If  inches 
in  diameter  and  1  inch  high  with  a  maximum  depth  of  bowl 
of  ^  inch.  A  bone-ash  cupel  of  these  dimensions  weighs  slightly 
more  than  40  grams  and  will  hold  the  litharge  from  a  30-gram 
button  with  but  little  leakage.  If  necessary  a  40-gram  button 
may  be  cupeled  in  it,  but  if  it  is  so  used  the  bottom  of  the  muffle 
should  be  well  covered  with  bone-ash.  A  bone-ash  cupel  will 
absorb  about  its  own  weight  of  litharge. 

Cupellation.  —  The  muffle  is  heated  to  a  light  red,  and  the 
cupels,  weighing  about  one-third  more  than  the  buttons  which 
are  to  go  into  them,  are  carefully  introduced  and  allowed  to  re- 
main for  at  least  ten  minutes,  in  order  to  expel  all  moisture  and 
organic  matter.  During  this  preliminary  heating  the  door  to 
the  muffle  is  ordinarily  kept  closed,  but  if  the  cupels  contain 
organic  matter  it  is  left  open  at  first  and  then  closed  for  five 
minutes  or  so  before  the  buttons  are  introduced. 

When  all  is  ready  the  buttons  are  placed  carefully  in  the 
cupels  and  the  muffle  door  again  closed.  If  the  cupels 
are  thoroughly  heated,  the  lead  will  melt  at  once  and  become 
covered  with  a  dark  scum.  If  the  temperature  of  the  muffle 
is  correct  this  will  disappear  in  the  course  of  a  minute  or  two 
when  the  molten  lead  will  become  bright.  The  assays  are  then 
said  to  have  opened  up  or  "  uncovered."  This  signifies 
that  the  lead  has  begun  to  oxidize  rapidly,  raising  the  temperature 
*  Eng.  and  Min.  Jour.  80,  p.  934. 


94  A   TEXTBOOK  OF  FIRE  ASSAYING 

of  the  molten  alloy  considerably  above  that  of  its  surroundings, 
whence  it  appears  bright.  It  assumes  a  convex  surface,  and 
molten  patches  of  litharge  passing  down  over  this  surface  give 
it  a  lustrous  appearance.  It  is  then  said  to  "  drive." 

When  the  assays  have  uncovered,  the  door  of  the  muffle  is 
opened  to  admit  a  plentiful  supply  of  air  to  promote  oxidation 
of  the  lead,  while  at  the  same  time  the  temperature  of  the  muffle 
should  be  reduced.  According  to  Fulton*,  if  the  buttons  are 
practically  pure  lead  the  temperature  of  uncovering  is  about 
850°  C.  However,  if  antimony,  cobalt,  nickel  etc.,  are  present, 
the  temperature  of  uncovering  and  also  that  required  for  cupella- 
tion  will  be  higher. 

The  greater  part  of  the  lead  oxide  formed  remains  liquid  and 
flows  down  over  the  convex  surface  of  the  molten  alloy.  If  the 
temperature  of  the  cupel  is  high  enough  this  molten  litharge  is 
absorbed.  A  small  part  of  the  lead  oxide  is  vaporized  and  ap- 
pears as  fume  rising  from  the  cupel. 

After  cupeling  has  proceeded  for  a  few  minutes,  a  ring,  caused 
by  the  absorbed  litharge,  may  be  seen  around  the  cupel  just 
above  the  surface  of  the  metal.  If  the  temperature  is  right  for 
cupeling  this  will  be  very  dull  red,  almost  black.  If  it  is  bright 
red,  the  temperature  is  too  high.  The  color  of  the  alloy  itself 
will  be  much  brighter  than  that  of  the  absorbed  litharge,  as  it 
is  in  fact  much  hotter  than  the  cupel  or  surrounding  air,  on  ac- 
count of  the  heat  generated  by  the  rapid  oxidation  of  the  lead. 
Next  to  the  formation  of  abundant  litharge  crystals,  the  appear- 
ance of  the  absorbed  litharge  is  the  best  indication  of  proper 
cupellation  temperature. 

If  the  temperature  is  exactly  right  feather-like  crystals  of 
litharge  form  on  the  sides  of  the  cupel  above  the  lead.  This 
is  due  to  sublimation  of  some  of  the  volatilized  lead  oxide.  In 
cupeling  for  silver  the  temperature  should  be  such  that  these  crys- 
tals are  obtained  on  at  least  the  front-half  of  the  cupel,  and  as 
the  button  grows  smaller  they  should  follow  it  down  the  side  of 
the  cupel,  leaving,  however,  a  slight  clear  space  around  it.  If 
the  temperature  becomes  too  low  for  the  cupel  to  absorb  the 
litharge,  the  crystals  begin  to  form  all  around  and  close  to  the 
lead  in  the  cupel,  and  soon  a  pool  of  molten  litharge  is  seen  form- 
ing all  around  the  annular  space  between  the  lead  and  the  cupel. 
*  Western  Chemist  and  Metallurgist,  4,  p.  31.  (Feb.  1908.) 


CUPELLATION  95 

If  the  temperature  of  the  cupel  is  not  quickly  raised,  this  pool 
increases  in  size  and  soon  entirely  covers  the  lead  and  then  solidi- 
fies. When  this  occurs  the  button  is  said  to  have  "  frozen," 
although  the  lead  itself  may  be  liquid  underneath.  Frozen  assays 
should  be  rejected  as  the  results  obtained  from  them,  by  again 
bringing  to  a  driving  temperature,  are  usually  low.  If  the  freez- 
ing is  noticed  at  the  start,  it  may  be  arrested  by  quickly  raising 
the  temperature  of  the  cupel  in  some  way,  i.  e.,  by  taking  away 
the  coolers,  closing  the  door  to  the  muffle,  opening  the  draft, 
putting  a  hot  piece  of  coke  in  front  of  the  cupel,  etc. 

Beginners  have  difficulty  in  noting  the  first  symptoms  of  freez- 
ing, but  all  should  be  able  to  see  the  pool  of  litharge  starting. 
This  gives  the  appearance  and  effect  of  oil;  if  the  cupel  is  moved 
the  button  slides  around  as  if  it  were  greased. 

Toward  the  end  of  the  cupellation  process  the  temperature 
must  be  raised  again,  because  the  alloy  becomes  more  difficultly 
fusible  as  the  proportion  of  silver  in  it  increases,  and  in  order  to 
drive  off  the  last  of  the  lead  a  temperature  of  about  900°  C.  should 
be  reached.  The  temperature  should,  not  be  raised  so  high  as 
to  molt  tho  crystals  of  litharge,  for  if  this  is  done  too  great  a  loss 
of  silver  results. 

As  the  alloy  becomes  richer  in  silver  it  becomes  more  and  more 
rounded  in  shape  and  shining  drops  of  litharge  appear  and  move 
about  on  its  surface.  As  the  last  of  the  lead  goes  off,  these  drops 
disappear,  the  fused  litharge  covering  becomes  very  thin  and, 
being  of  variable  thickness,  gives  an  effect  of  interference  of 
light,  so  that  the  bead  appears  to  revolve  and  presents  a  succes- 
sion of  rainbow  colors.  This  phenomenon  is  termed  the  "  play  of 
colors."  The  colors  disappear  shortly,  the  bead  becomes  dull  and 
after  a  few  seconds  appears  bright  and  silvery.  ^This  last  change 
is  called  the  "  brightening." 

After  brightening,  the  cupels  should  be  left  in  the  furnace  for 
a  few  minutes  to  ensure  removal  of  the  last  of  the  lead,  and  then 
moved  gradually  to  the  front  of  the  muffle  before  they  are  taken 
out,  so  that  cooling  may  be  slow. 

As  the  bead  solidifies  it  will  "  flash  "  or  "  blick,"  i.  e.,  suddenly 
emit  a  flash  of  light  due  to  the  release  of  the  latent  heat  of  fusion, 
which  raises  the  temperature  very  much  for  a  short  time. 

Cupels  containing  large  silver  beads  should  be  drawn  to  the 
front  of  the  muffle  until  they  chill.  Just  as  the  bead  is  about  to 


96  A   TEXTBOOK  OF  FIRE  ASSAYING 

solidify  a  very  hot  cupel  is  placed  over  them  and  allowed  to  stand 
for  several  minutes,  after  which  they  are  slowly  withdrawn  from 
the  muffle.  The  hot  cupel  melts  the  outside  crust  of  solid  silver 
and  causes  solidification  to  go  on  from  below.  If  this  precau- 
tion is  not  taken,  the  beads  may  "  sprout  "  or  "  spit."  This 
action  is  caused  by  the  sudden  escape  of  oxygen  which  is  dissolved 
in  the  molten  silver  and  expelled  when  the  bead  solidifies.  If 
the  bead  is  allowed  to  solidify  rapidly,  a  crust  of  solid  silver  forms 
on  the  outside,  and  as  the  central  part  solidifies  this  crust  is 
violently  ruptured  by  the  expelled  oxygen,  giving  a  cauliflower- 
like  growth  on  the  bead  and  causing  particles  of  silver  to  be 
thrown  off.  As  a  consequence  the  results  obtained  from  sprouted 
beads  are  unreliable.  Beads  containing  one-third  or  more  of 
gold  will  not  sprout  even  if  rapidly  withdrawn  from  the  muffle. 
Sprouting  is  said  to  be  an  evidence  of  the  purity  of  the  silver. 

The  silver  bead  should  appear  smooth  and  brilliant  on  the 
upper  surface,  and  should  be  silver-white  in  color  and  spherical 
or  hemispherical  in  shape,  according  to  its  size.  It  should  adhere 
slightly  to  the  cupel  and  appear  frosted  on  the  under  surface. 
If  the  bead  is  smooth  on  the  bottom  and  does  not  adhere  to  the 
cupel,  it  is  an  indication  of  too  low  a  finishing  temperature.  Such 
a  bead  will  always  contain  lead.  If  it  has  rootlets  which  extend 
into  cracks  of  the  cupel  the  results  are  also  to  be  taken  as  unreli- 
able, as  some  of  the  silver  may  be  lost  in  the  cupel. 

Lead  buttons  very  rich  in  gold  and  silver  have  a  peculiar  mottled 
appearance  after  cupeling  begins.  Oily  drops  of  litharge  appear 
and  move  about  on  the  surface  of  the  alloy  and  finally  run  down 
the  side  of  the  convex  surface  and  are  absorbed  by  the  cupel. 
This  appearance  is  characteristic  and  once  seen  is  easily  recog- 
nized again.  It  may  be  seen  toward  the  end  of  cupellation  with 
any  alloy  containing  much  precious  metal  and  is  an  indication 
of  the  approach  of  the  end  and  a  reminder  that  the  temperature 
should  be  raised  to  ensure  driving  off  the  last  of  the  lead. 

The  minimum  temperature  at  which  cupellation  will  proceed 
has  been  a  more  or  less  disputed  point,  owing  largely  to  a  differ- 
ence in  conception  of  the  process  and  involved  conditions.  At 
least  three  methods  of  measuring  the  temperature  have  been 
proposed.  One  experimenter  held  his  pyrometer  junction  one- 
quarter  inch  above  the  alloy  in  the  cupel,  another  placed  the 
junction  inside  the  cupel,  while  a  third  measured  the  temperature 


CVPELLATION  97 

of  the  alloy  itself.  According  to  Fulton*  the  alloy  itself  must 
be  between  800  and  850°  C.  Litharge  melts  at  884°  (Mosto- 
witch),  906°  (Bradford),  but  passes  through  a  pasty  stage  before 
becoming  liquid.  It  would  seem  that  the  cupel  itself  must  be 
maintained  above  the  melting  point  of  litharge  in  order  to  allow 
of  absorption.  At  any  event  the  cupel  is  much  hotter  than  the 
space  around  it  partly  because  of  the  heat  generated  by  the  oxida- 
tion of  the  lead  and  partly  because  the  cupel  rests  on  the  floor 
of  the  muffle  and  its  interior  portion  becomes  heated  by  conduc- 
tion through  the  muffle  floor  on  which  it  stands.  Bradford  f 
found  906°  C.  to  be  the  minimum  cupel  temperature  which  would 
permit  of  absorption  of  litharge.  Lodge  \  found  that  for  silver 
cupellation  with  a  moderate  draft,  the  muffle  temperature  (taken 
one-quarter  inch  above  the  cupels)  should  be  between  650°  and 
700°  C. 

FIRST  EXERCISE.     PRACTICE   IN    CUPELLATION. 

Procedure.- — Take  from  0.10  to  0.20  grams  of  silver,  but  do 
not  waste  time  in  weighing.  Wrap  in  25  to  30  grams  of  sheet 
lead.  Prepare  two  or  three  of  these  portions  and  cupel  one  at 
a  time  in  order  to  become  familiar  with  the  operation,  and  with 
the  correct  temperature.  To  study  the  end  phenomena  "  play  of 
colors/'  "  brightening,"  "  blick  "  etc.,  the  same  or  a  larger  amount 
of  silver  may  be  used  with  a  smaller  amount  of  lead,  say  10  grams. 

Have  the  muffle  at  a  bright  red;  be  sure  that  the  cupels  are 
dry  and  then  heat  gradually  until  they  are  red.  Allow  at  least 
ten  minutes  for  this.  Be  sure  that  the  cupels  weigh  more  than 
the  lead,  and  that  the  bowl  is  sufficiently  large  to  contain  the 
melted  alloy.  Have  a  row  of  extra  cupels  in  front  of  those  which 
are  to  be  used  and  keep  them  there  throughout  the  process. 
Keep  the  door  to  the  muffle  closed  and  when  the  cupel  is  red 
throughout  and  heated  to  about  850°  C.  place  the  packet  of  lead 
and  silver  carefully  in  the  cupel  and  close  the  door  to  the  muffle 
so  that  the  lead  will  fuse  as  quickly  as  possible.  As  soon  as  the 
assay  begins  to  "  drive,"  note  the  time,  open  the  door  of  the 
muffle  and  lower  the  temperature  of  the  cupel  by  checking  the 
fire  and  by  placing  cold  scorifiers,  etc.,  around  it.  Continue  to 
reduce  the  temperature  until  feather  crystals  of  litharge  are 

*  Western  Chemist  and  Metallurgist,  4,  p.  31  (1908). 
f  Jour.  Ind.  and  Eng.  Chem.  1,  p.  181. 
t  Notes  on  Assaying,  p.  62. 


98  A   TEXTBOOK  OF  FIRE  ASSAYING 

seen  forming  at  least  on  the  front  half  of  the  cupel.  Then  con- 
tinue the  cupellation  at  this  temperature.  Finally  finish  the 
assay  at  a  somewhat  higher  heat,  increasing  the  temperature 
by  starting  up  the  fire,  removing  the  coolers  or  by  shutting  off 
some  of  the  cold  air  supply  by  partly  closing  the  door  to  the 
muffle.  If  the  cupels  are  running  very  cold  it  will  be  necessary 
to  start  raising  the  temperature  some  five  minutes  before  the 
end.  The  fire  should  be  under  good  control  at  all  times.  As 
soon  as  the  cupellation  is  finished  remove  the  assay  carefully 
from  the  muffle  to  avoid  sprouting.  All  assay ers  agree  that 
the  best  results  are  obtained  by  having  a  hot  start,  a  cold  drive, 
and  a  higher  heat  again  at  the  finish. 

Notes:  1.  When  a  number  of  cupellations  are  carried  on  at  one  time, 
the  buttons  should  be  charged  in  the  order  of  their  size,  i.e.,  largest  first,  so 
that  all  may  start  driving  together. 

A  skilful  assayer,  with  a  large  muffle,  can  run  as  many  as  fifty  cupellations 
at  one  time  and  obtain  feather  crystals  on  all. 

2.  When  a  number  of  cupellations  are  carried  on  at  one  time,  the  cupels 
are  not  moved  about  after  the  lead  is  put  in,  but  the  temperature  is  regulated  by 
means  of  the  draft  and  firing  and  by  the  use  of  coolers,  (cold  scorifiers,  cupels, 
crucible  covers,  etc.)  which  are  put  in  toward  the  back  of  the  furnace  and 
replaced  as  soon  as  they  become  heated. 

3.  Bear  in  mind  that  although  the  temperature  of  the  muffle  may  be  as 
low  as  650°  or  700°  C,  the  cupel  itself  should  be  slightly  above  the  freezing 
point  of  litharge,  to  allow  of  its  being  absorbed.     It  has  been  found  best, 
therefore,  to  protect  the  body  of  the  cupel  itself  from  the  draft  through  the 
muffle,  by  placing  an  extra  row  of  cupels  or  a  low  piece  of  fire-brick  in  front 
of  the  first  row  of  cupels. 

4.  Buttons  containing  copper  may  be  cupeled  at  a  lower  temperature 
than  those  consisting  of  pure  lead  and  silver,  owing  to  the  fact  that  cupric 
oxide  lowers  the  freezing  point  of  litharge. 

5.  After  the  cupellation  is  finished,  the  cupel  should  be  left  in  the  muffle 
one  or  two  minutes,  depending  on  the  size  of  the  bead,  to -remove  the  last 
traces  of  lead.     After  this  it  should  be  withdrawn;   otherwise  a  loss  of  silver 
ensues. 

6.  When  the  finishing  temperature  is  too  low,  the  beads  will  solidify  with- 
out brightening.     They  retain  lead  and  have  a  dull  appearance  and  some- 
times show  flakes  of  litharge  on  the  surface.     Under  certain  conditions  they 
flatten  out,  leaving  a  gray,  mossy  bead. 

7.  When  the  button  contains  only  gold,  a  higher  finishing  temperature 
is  required  than  when  working  for  silver. 

8.  When  gold  is  present  in  considerable  amounts  the  bead  will  not  sprout 
even  if  taken  directly  out  of  the  muffle. 

9.  Besides  gold  and  silver,  the  bead  may  contain  platinum,  palladium, 
rhodium,  iridium,  ruthenium,  osmium,  and  iridosmium. 


CUPELLATION  99 

10.  If  the  upper  surface  of  the  bead  appears  to  be  frosted  this  indicates 
the  presence  of  tellurium  or  some  member  of  the  platinum  group. 

11.  Buttons  which  contain  a  large  amount  of  platinum  flatten  out  arid 
will  not  blick.     They  have  a  steel-gray  color  and  a  dull  surface. 

SECOND   EXERCISE.     CUPELLATION   ASSAY   OF 
LEAD  BULLION. 

Procedure.  —  Weigh  out  carefully  three  portions  of  bullion 
of  \  A.  T.  each.  Wrap  each  in  10  to  15  grams  of  silver-free  lead 
foil  so  that  the  whole  is  very  compact,  having  each  piece  of  lead 
foil  of  the  same  size  and  weight. 

Have  a  good  fire  so  that  the  lead  will  melt,  and  start  to  drive 
without  delay.  Use  cupels  which  weigh  35  grams  or  more  and 
have  them  all  in  a  row  with  an  extra  row  in  front.  Drop  the 
assays  in  as  quickly  as  possible  and  close  the  door.  As  soon  as 
the  lead  starts  to  drive,  close  the  drafts  and  cool  as  soon  as  possible 
so  that  feather  crystals  of  Ktharge  form  on  at  least  the  front 
half  of  the  cupel.  Finally  open  the  draft  and  otherwise  increase 
the  temperature  for  the  last  minute  or  two  of  cupellation  to  drive 
off  the  last  traces  of  lead.  Have  some  hot  cupels  in  the  muffle 
and,  as  soon  as  the  beads  brighten,  pull  them  forward  in  the 
muffle  to  chill  and  then  put  a  hot  cupel  over  them  and  withdraw 
both  slowly  from  the  muffle.  All  danger  of  sprouting  is  over 
when  the  inside  of  the  cupel  reaches  a  dull  red  or  when  the  bead 
has  become  solid  throughout.  Remove  from  the  furnace  to  the 
cupel  tray  and  allow  to  cool.  When  the  bead  is  cold,  detach  it 
from  the  cupel  with  the  pliers  and  brush  with  a  stiff  brush  to 
remove  bone-ash,  or  place  it  on  its  side  on  a  clean  anvil  and 
slightly  flatten  with  a  hammer.  When  the  bead  is  free  from 
bone-ash,  weigh  it,  recording  in  the  notebook  the  weight  of  gold 
and  silver.  Then  part  and  weigh  the  gold;  finally  report  the 
amount  of  gold  and  silver  in  ounces  per  ton. 

Notes:  1.  Have  a  sheet  of  clean  white  paper  at  hand  and  when  trans- 
ferring the  bullion  from  the  scale-pan  to  the  lead  foil  do  it  over  this  so  that  in 
case  any  is  spilled  it  will  be  seen  and  recovered.  Do  all  of  the  wrapping  and 
compressing  over  this  paper  for  the  same  reason. 

2.  If  the  assay  is  not  compact,  it  may  overflow  the  cupel  while  melting, 
or  else  leave  small  particles  on  the  sides  of  the  cupel,  which  will  not  come 
down  into  the  main  button. 

Loss  of  Silver  in  Cupeling.  —  There  is  always  some  loss  in 
cupellation,  the  amount  depending  on  many  factors  such  as  the 
nature  and  shape  of  the  cupel,  the  temperature  of  cupellation, 


100 


A    TEXTBOOK  OF  FIRE  ASSAYING 


the  proportion  of  lead  to  silver,  the  amount  and  character  of 
impurities,  the  draft  through  the  muffle,  etc.  Losses  may  be 
due  to  spurting,  absorption  of  bullion  by  the  cupel,  oxidation 
and  absorption  of  silver  with  litharge,  and  volatilization  of  sil- 
ver either  alone  or  accompanied  by  other  metals. 

The  cupel  surface  may  be  regarded  as  a  membrane  permeable 
to  molten  litharge  and  impermeable  to  lead.  The  more  nearly 
the  material  of  the  cupel  surface  approaches  this  condition  the 
lower  the  losses  may  be  made.  Some  cupels,  particularly  some  of 
magnesite,  present  spots  of  material  which  are  permeable  to  lead 
and  consequently  give  a  high  loss  of  silver. 

The  most  important  factor  relative  to  cupel  loss,  however, 
is  the  temperature.  The  higher  the  temperature,  the  higher 
the  loss,  is  an  invariable  rule.  The  increased  loss  due  to  higher 
temperature  seems  to  be  due  mostly  to  an  increased  oxidation  of 
the  silver  and  a  consequent  greater  absorption  loss.  The  volatil- 
ization loss  is  also  increased  by  an  increase  of  temperature.  A 
loss  of  1  per  cent  silver  is  allowable  and  the  loss  may  usually  be 
kept  close  to  this  figure  by  taking  pains  to  cupel  with  abundant 
crystals  of  litharge.  If  this  matter  is  overlooked  a  loss  of  4  or  5 
per  cent  may  readily  be  obtained  and  this,  of  course,  is  entirely 
inadmissable. 

The  following  table,  taken  from  Lodge's  "Assaying,"  illustrates 
this  point  and  shows  the  importance  of  cupeling  at  the  correct 
temperature.  The  temperature  was  taken  with  a  Le  Chatelier 
pyrometer,  the  junction  being  held  about  one-quarter  inch  above 
the  button. 

TABLE  IX. 
EFFECT  OF  TEMPERATURE  ON  Loss  OF  SILVER  IN  CUPELLATION. 


Tempera- 

Silver 

Remarks 

Silver 
milligrams 

Lead 

grams 

ture 
degrees 

loss 

centigrade 

200 
200 

10 
10 

700 

775 

1.02 
1.30 

Crystals  of  PbO  all  around 
Crystals  of  PbO  on  cooler 

button, 
side  of 

cupel. 

200 

10 

850 

1.73 

No  crystals. 

200 

10 

925 

3.65 

U               (I 

200 

10 

1000 

4.88 

11            ft 

Average  figures. 


CUPELLATION 


101 


The  amount  of  lead  and  silver  present  in  any  button  has  a 
marked  effect  on  the  percentage  loss  of  silver  in  cupellation. 
Rose,*  in  speaking  of  cupellation  says,  "  The  losses  of  silver  at 
first  are  small,  so  long  as  large  quantites  of  base  metals  protect 

it  from  oxidation Later,  when  the  percentage  of  silver  is  high 

it  is  freely  oxidized  ....  and  the  oxidation  is  at  its  maximum  when 
the  silver  is  practically  pure." 


PROGRESSIVE  LOSS  OF  SILVER 
DURING  CUPELLATION 


0 
30 


5  10  15  20 

Time  of  Cupdlihq  Minutes 
25  20  15  10 

Grams  of  Lead  Remaining 

FIG.  40.  —  Curve  showing  cumulative  loss  of  silver  in  cupellation. 


This  is  well  illustrated  by  the  curve  shown  in  Fig.  40  in  which 
is  plotted  the  cumulative,  minute-to-minute,  loss  of  silver  in  cupel- 
ing a  30-gram  button  containing  100  milligrams  of  silver.  This 
is  the  result  of  considerable  careful  experimental  work  done  in 
the  fire  assay  laboratory  of  the  Massachusetts  Institute  of  Tech- 
nology, several  years  ago. 

Keeping  the  amount  of  silver  constant  and  varying  the  lead, 
Lodge  obtains  the  results  shown  in  the  following  table : 

*  Trans.  Inst.  Min.  Met.,  14,  p.  420. 


102 


A   TEXTBOOK  OF  FIRE  ASSAYING 


TABLE  X. 

EFFECT  OF  LEAD  ON  Loss  OF  SILVER  IN  CUPELLATION. 


Silver 

Lead 

Temperature 

Silver  loss 

milligrams 

grams 

degrees  centigrade 

per  cent1 

200 

10 

685 

1.39 

200 

15 

685 

1.38 

200 

20 

685 

1.52 

200 

25 

685 

1.85 

1  Average  of  two  nearest  together. 

When  the  quantity  of  lead  remains  constant  and  the  silver 
is  varied  the  percentage  loss  of  silver  is  found  to  increase  as  the 
silver  is  reduced.  The  following  representative  figures  taken 
from  Godshall's  paper  on  "  Silver  Losses  in  Cupellation  "*  show 
this  very  clearly. 

TABLE  XI. 
EFFECT  OF  VARYING  SILVER  ON  CUPELLATION  LOSSES. 


Weight  of  Lead 

1/2  A.  T. 

1/2  A.  T. 

1/2  A.  T. 

1/2  A.  T. 

1/2  A.  T. 

1/2  A.  T. 

1/2  A.  T. 

Weight  of  Silver 

200  mg. 

100  mg. 

50  mg. 

20  mg. 

10  mg. 

5  mg. 

2  mg. 

Silver  Loss 

1.73% 

2.03% 

2.65% 

2.82% 

3.44% 

4.46% 

6.90% 

Loss  of  Gold  in  Cupeling.  —  There  is  always  some  loss  of  gold 
in  cupeling,  but  owing  to  the  greater  resistance  of  this  metal  to 
oxidation  this  loss  is  smaller  than  the  corresponding  silver  loss. 
The  following  table,  taken  from  Lodge,  shows  the  relation  be- 
tween the  loss  of  gold  and  the  temperature  of  cupellation. 

TABLE  XII. 

EFFECT  OF  TEMPERATURE  ON  Loss  OF  GOLD  IN  CUPELLATION. 


Tempera- 

Gold used 
milligrams 

Lead 
grams 

ture 
degrees 

Gold  loss 
per  cent1 

Remarks 

centigrade 

200 

10 

700 

Button  froze. 

200 

10 

775 

0.155 

200 

10 

850 

0.385 

200 

10 

925 

0.460 

200 

10 

1000 

1.435 

200 

10 

1075 

2.990 

1  Mean  of  two  results  nearest  together. 
*  Trans.  A.I.M.E.  26,  pp.  473-484  inc. 


CUPELLATION  .    103 

In  the  case  of  the  gold  with  temperatures  of  1000  degrees  and 
above,  the  higher  losses  seem  to  be  due  in  part  to  a  lessening 
of  the  surface  tension  owing  to  the  increased  temperature,  for 
when  the  cupels  were  examined  with  the  microscope  a  large  num- 
ber of  minute  beads  were  found  all  over  the  inner  surface.  It 
would  appear  that  small  particles  of  the  alloy  were  left  behind 
to  cupel  by  themselves. 

As  in  the  case  of  silver,  the  percentage  loss  of  gold  is  found 
to  increase  as  the  quantity  is  reduced.  Hillebrand  and  Allen  * 
show  that,  contrary  to  the  usual  opinion,  the  loss  of  gold  in  cupel- 
ing is  not  negligible,  and  is  greatly  influenced  by  slight  changes 
in  temperature  They  found  that  the  most  exact  results  were  ob- 
tained when  feather  crystals  of  litharge  were  obtained  on  the  cupels. 

Effect  of  Silver  on  the  Loss  of  Gold  in  Cupeling.  —  Lodge,  in 
his  "  Notes  on  Assaying,"  states  that  the  addition  of  silver  in 
excess  lessens  the  loss  of  gold,  but  gives  no  figures.  Hillebrand 
and  Allen  f  state  that  the  loss  of  gold  in  cupeling  is  greater  with 
pure  gold  and  alloys  poor  in  silver  than  with  alloys  rich  in  silver. 
Smith  t  gives  the  following  figures  showing  the  protective  action 
exercised  by  silver  on  gold  during  cupellation : 

Per  cent  of  total  gold  recovered. 
Tellurium  added.  Without  tellurium. 

Without  silver  94 . 9  98 . 2 

With  silver  97.0  99.5 

In  order  to  obtain  more  light  upon  this  subject,  Mr.  A.  B. 
Sanger,  a  student  at  the  Massachusetts  Institute  of  Technology, 
made  a  large  number  of  careful  experiments,  using  proof  gold 
and  C.  P.  silver.  The  work  was  done  in  a  gas  furnace  and  the 
temperature  was  measured  by  a  thermo-electric  pyrometer,  the 
junction  of  which  was  placed  in  the  center  of  a  blank  cupel  in 
line  with  the  cupels  which  were  being  used.  Mr.  Sanger  used 
10  milligrams  of  gold  and  various  amounts  of  silver  with  25 
grams  of  lead,  and  with  four  different  temperatures.  His  results  § 
are  shown  in  Fig.  41.  The  gold  losses  include  any  solution  losses 
there  may  have  been  in  parting,  but  these  are  extremely  small, 
or  nil.  The  results  show  a  very  decided  protective  effect  of  sil- 

*  Bull.  No.  253  U.  S.  Geol.  Survey,  p.  20  et  seq. 
t  Op.  cit. 

J  The  Behavior  of  Tellurium  in  Assaying.  Trans.  Inst.  Min.  Met.  17, 
p.  472. 

§  Thesis  No.  492,  M.I.T.  Mining  Department. 


104 


A   TEXTBOOK  OF  FIRE  ASSAYING 


35 


5-5 


II 

cx    & 


}UQO  J9J 


CUPELLATION 


105 


ver  and  confirm  the  statements  of  Lodge  and  others.  A  glance 
at  the  curves  shows  how  important  it  is  to  run  gold  assays  at  a 
temperature  close  to  that  at  which  feather  crystals  are  obtained. 
With  smaller  amounts  of  gold  the  percentage  losses  will  be  corre- 
spondingly greater. 

Influence  of  Impurities  on  the  Loss  of  Precious  Metals  during 
Cupellation.  —  According  to  Rose,*  tellurium,  selenium,  thal- 
lium, bismuth,  molybdenum,  manganese,  copper,  vanadium,  zinc, 
arsenic,  antimony,  cadmium,  iron  and  tin,  all  induce  extra  losses 
of  gold  and  silver  in  cupellation  and  should  be  removed  before 
this  stage  is  reached. 

The  behavior  of  tellurium  in  cupellation  will  be  mentioned 
in  the  discussion  of  the  assay  of  telluride  ores.  Copper  is  per- 
haps the  most  common  impurity,  and  on  account  of  the  difficulty 
of  removing  it  completely  in  scorification  or  crucible  fusions,  a 
knowledge  of  its  behavior  in  cupeling  is  particularly  important. 
Eager  and  Welch  f  give  the  following  table  showing  the  effect  of 
copper  on  the  loss  of  silver  in  cupellation. 

TABLE  XIII. 
EFFECT  OF  COPPER  ON  SILVER  LOSSES  IN  CUPELLATION. 


No. 

Silver 
used 

grams. 

Lead 

grams 

Tempera- 
ture 
degrees 
centigrade 

Copper 
per  cent 
of  the 
silver 

Per  cent  silver 
lost 

Ratio  of 
lead  to 
copper 

Individual 

Mean 

1 

.20382 

10 

775 

5 

1.00 

1000  to  1 

2 

.20256 

u 

1.15 

M 

3 

.20036 

" 

0.93 

1.03 

•     " 

4 

.20618 

10 

1.19 

500  to  1 

5 

.20193 

ii 

1.09 

" 

6 

.20118 

u 

1.06 

1.11 

«    • 

7 

.20146 

15 

1.35 

333  to  1 

8 

.20138 

M 

1.27 

« 

9 

.20432 

«« 

»  1.15 

1.31 

ii 

10 

.20282 

20 

11.15 

250  to  1 

11 

.20100 

11 

1.45 

u 

12 

.20338 

" 

1.46 

1.46 

" 

13 

.20224 

25 

1.05 

200  to  1 

14 

.20496 

" 

0.95 

<< 

15 

.20420 

1.07 

1.02 

ii 

1  Disregarded. 

*  Jour.  Chem.  Met.  and  Min.  Soc.  of  South  Africa,  6,  p.  167. 
f  Thesis  No.  225,  M.I.T.  Mining  Department. 


106 


A   TEXTBOOK  OF  FIRE  ASSAYING 


When  the  results  shown  in  this  table  are  compared  with  those 
in  Table  IX,  it  appears  that  the  presence  of  a  small  amount  of  cop- 
per, not  more  than  1  part  to  500  of  lead,  and  not  more  than  1 
part  to  10  parts  of  silver,  reduces  the  loss  of  silver  below  that 
which  results  when  no  copper  is  used.  This  may  be  due  to  the 
protective  action  which  copper  is  known  to  exert  upon  silver.* 

With  a  ratio  of  1  part  of  copper  to  333  parts  of  lead  and  with 
6.67  parts  of  silver,  the  loss  is  about  the  same  as  in  the  absence 
of  copper. 

With  an  increase  in  the  amount  of  copper  to  1  part  to  250  of 
lead  and  5  parts  of  silver  the  loss  is  greater  than  when  no  copper 
is  used. 

With  a  ratio  of  1  part  of  copper  to  200  parts  of  lead  and  4  parts 
of  silver  the  loss  apparently  becomes  less,  but  this  was  found  to 
be  due  to  the  retention  of  copper  in  the  silver  bead. 


TABLE  XIV. 
EFFECT  OF  COPPER  ON  GOLD  LOSSES  IN  CUPELLATION. 


No. 

Gold 
used 
grams 

Lead 

grams 

Tempera- 
ture 
degrees 
centigrade 

Copper 
per  cent 
of  the 
gold 

Per  cent  gold  lost 

Ratio  of 
lead  to 
copper 

Individual 

Mean 

1 

.20181 

10 

775 

None 

0.15 

2 

.20104 

" 

0.16 

0.16 

3 

.20288 

5 

0.18 

1000  to  1 

4 

.20110 

tt 

0.20 

tt 

5 

.20318 

" 

0.10 

tt 

(In  the  following  the  beads  show  a  gain  in  weight.) 

6 

.20102 

10 

-0.03 

500  to  1 

7 

.20142 

tt 

-0.03 

tt 

8 

.20138 

(i 

-0.02 

-0.03 

1C 

9 

.20024 

15 

-0.11 

333  to  1 

10 

.20060 

tt 

-0.26 

tt 

11 

.20048 

" 

-0.18 

-0.18 

ti 

12 

.20100 

20 

-0.13 

250  to  1 

13 

.20101 

tt 

-0.561 

ft 

14 

.20161 

11 

-0.20 

-0.17 

" 

15 

.20422 

25 

-0.29 

200  to  1 

16 

.20296 

tt 

-0.21 

tt 

17 

.20284 

tt 

-0.32 

-0.27 

it 

1  Disregarded. 
*  Rose,  Trans.  Inst.  Min.  Met.  14,  p.  422. 


CVPELLATION  107 

When  the  amounts  of  lead  and  copper  in  10,  11  and  12  above 
are  compared,  it  is  found  that  the  ratio  of  lead  to  copper  should 
be  at  least  250  to  1  to  ensure  the  removal  of  the  copper,  and  at 
least  333  to  1  if  the  apparent  loss  of  silver  is  not  to  be  noticeably 
increased. 

The  effect  of  copper  on  the  loss  of  gold  is  shown  in  the  preceding 
table. 

It  appears  that  5  per  cent  of  copper  with  this  lead  ratio  has  no 
effect  on  the  loss  of  gold.  The  gain  in  the  weight  of  the  gold 
beads  with  10  per  cent  and  over  of  copper  shows  clearly  that 
copper  is  retained  by  the  gold  under  these  conditions.  This 
was  also  indicated  by  the  color  of  the  gold  beads.  With  a  higher 
cupellation  temperature  the  amount  of  copper  retained  would 
doubtless  be  smaller.  It  is  interesting  to  note  that  with  10  per 
cent  of  copper  the  amount  retained  by  the  bead  approximately 
neutralizes  the  loss  of  the  gold  itself.  Apparently  the  ratio  of 
lead  to  copper  should  not  be  less  than  500  to  1  if  the  copper  is 
to  be  completely  removed. 

Rule  Governing  Cupellation  Losses.  —  As  is  well  recognized 
in  large-scale  cupeling  operations,  the  concentration  of  precious 
metal  in  the  litharge  increases  as  the  concentration  in  the  lead 
increases.  W.  J.  Sharwood*  after  examining  a  large  number  of 
experimental  results  enunciated  the  following  empirical  rule  con- 
necting the  actual  or  percentage  loss  with  the  weight  of  the  bead: 
"  When  a  given  amount  of  silver  (or  of  gold)  is  cupeled  with  a 
given  amount  of  lead,  under  a  fixed  set  of  conditions  as  to  tem- 
perature, etc.,  the  apparent  loss  of  weight  sustained  by  the  prec- 
ious metal  is  directly  proportional  to  the  surface  of  the  bead  of 
fine  metal  remaining." 

If  the  above  is  true  the  following  are  also  true. 

(1)  "  The  loss  of  weight  varies  as  the  f  power  of  the  weight, 
or  as  the  square  of  the  diameter  of  the  bead." 

(2)  "  The  percentage  loss  varies  inversely  as  the  diameter  of 
the  bead,  or  inversely  as  the  cube  root  of  the  weight." 

As  Sharwood  points  out  it  might  be  better  to  base  the  calcula- 
tions on  the  original  weight  of  metal  taken,  but  in  every  day  prac- 
tice this  is  not  known  and  we  have  to  depend  on  the  weight  of 
the  bead. 

Inasmuch  as  small  variations  in  the  amount  of  lead  have  but 

*  T.A.I.M.E.  52,  p.  180. 


108 


A   TEXTBOOK  OF  FIRE  ASSAYING 


L 

1 

\ 

\ 

\ 

\ 

1 

\ 

\ 

\ 

)' 

\ 

\ 

\ 

-Q\ 

CQ 

1 

\ 

\ 
\                  N 

1 

\ 

\ 

'      ^ 

\ 

\ 

\ 

\ 

\ 

« 

v\ 

\ 

e 

K 

A 

\ 

\ 

1 

° 

y 

7        c5 

\ 

\ 

\ 

\ 

1 

\ 

\ 
\ 

_!_ 

\ 

\ 

\ 
\ 
\ 
\ 

i 

\ 

\ 

\ 

\ 

1 

\ 

\ 

1 

\ 

\ 

oo      <to  ^ 

C>        C>  <S 

'ISO!  U3A1IS 


CM 


CUPELLATION  109 

little  effect  on  the  cupellation  loss,  and  as  temperature  condi- 
tions in  a  given  row  across  a  muffle  are  nearly  uniform,  we  may 
apply  this  rule  to  determine  the  proper  correction  for  a  bead  of 
any  weight  by  a  calculation  applied  to  the  loss  observed  in  a 
proof  of  an  entirely  different  weight,  but  cupeled  at  the  same 
time  under  the  same  conditions.  The  simplest  method  of  finding 
the  amount  of  this  correction  for  any  particular  case  is  by  the 
use  of  a  logarithmic  plot  of  the  equation  y  =  Cxu,  which  is  a 
straight  line,  and  on  which,  when  the  horizontal  and  vertical 
scales  are  equal,  the  tangent  made  by  the  line  and  the  x  axis  is 
the  exponent  n. 

In  Fig.  42  is  plotted  the  equation  y  =  l/10z*  when  the 
abscissa^  equals  the  milligrams  of  silver  cupeled  and  the  ordinate 
y  equals  the  silver  loss  in  milligrams.  To  find  a  correction  run 
one  proof,  as  near  the  expected  weight  as  possible  in  each  row 
and  plot  the  corresponding  point  (a)  on  the  diagram,  and  draw 
through  it  a  line  parallel  to  the  guide  line  y  =  l/10z*.  Note 
the  points  on  the  line  corresponding  to  the  weights  B,  C,  and  D 
of  other  beads  weighed,  and  read  the  correction  for  each  from 
the  scale. 

Dewey,  in  discussing  Sharwood's  results,  warns  against  placing 
too  much  reliance  on  this  rule,  however,  because  as  he  says, 
"  It  is  so  easy  to  say  '  if  all  other  conditions  remain  the  same  ' 
but  it  is  so  extremely  difficult,  and  in  practical  work  impossible, 
to  actually  maintain  equal  conditions." 

Testing  Cupels  for  Absorption  of  Silver.  —  An  occasional  test 
of  cupels  and  especially  of  each  new  lot  of  bone-ash  is  desirable. 
Select  some  standard  amount  of  lead  and  silver  and  always 
use  the  same  amounts  so  that  results  may  be  comparable.  One 
hundred  milligrams  of  silver  and  25  grams  of  lead  is  a  convenient 
quantity.  i 

Indications  of  Metals  Present.  —  The  lead  buttons  obtained 
from  the  assay  of  ores  ordinarily  give  but  little  trouble  in  cupel- 
lation but  occasionally  the  ore  may  contain  some  unsuspected 
impurity  which  makes  its  appearance  during  the  cupellation  pro- 
cess. In  addition  to  all  sorts  of  ores  the  assayer  often  has  sub- 
mitted to  him  various  kinds  of  bullion,  numerous  by-products 
of  the  mining  and  metallurgical  industries,  as  well  as  such  material 
as  jeweler's  sweeps,  dental  alloys,  etc.  All  of  the  latter  may  and 
do  usually  contain  considerable  amounts  of  base-metal  impurities 


110  A    TEXTBOOK  OF  FIRE  ASSAYING 

as  well  as  occasional  rare  metals,  many  of  which  may  exert  con- 
siderable influence  on  the  results  of  the  assay  if  provision  is  not 
made  for  them.  It  is  always  important  to  determine  the  nature 
of  the  constituents  of  the  material  which  is  being  assayed;  and 
the  behavior  of  the  cupeling  lead  and  the  appearance  of  the  cupel 
and  bead  during  and  after  cupellation  will  often  give  much  valu- 
able information  concerning  the  elements  present.  When  the 
character  of  the  constituents  is  ascertained,  the  skilful  assayer 
will  know  exactly  what  to  do,  and  when  he  again  comes  to  the 
cupellation  stage  everything  will  go  smoothly.  All  that  is  known 
about  qualitative  analysis  is  not  found  in  books  devoted  to  that 
subject,  as  assay ers  well  know;  and  in  the  case  of  certain  rare 
elements  at  least,  the  fire  assayer  has  a  decided  advantage  over 
the  ordinary  chemist. 

As  soon  as  the  button  melts  any  slag  which  may  not  have  been 
removed  in  cleaning  it,  together  with  sulphides  or  arsenides  of 
some  of  the  base  metals,  if  present,  will  come  to  the  surface 
of  the  alloy  as  a  dark-colored  pasty  dross.  If  not  too  great  in 
amount  this  will  go  to  the  side  of  the  cupel  and  cupellation  may 
be  continued.  If  zinc,  tin,  iron,  nickel,  cobalt,  antimony  or  ar- 
senic are  present  these  will  oxidize  and  come  off  in  the  order 
named.  The  oxides  of  zinc,  tin,  iron,  nickel  and  cobalt  are  but 
slightly  soluble  in  molten  litharge  and,  if  present  in  any  consider- 
able amount,  give  infusible  scoria  which  float  on  top  of  the  lead 
and  interfere  with  cupellation. 

ZINC  if  present  will  burn  with  a  brilliant  greenish-white 
flame  and  emit  dense  white  fumes.  A  considerable  part  of  the 
oxide  condenses  on  the  cupel  and  may  cover  over  the  lead  thus 
preventing  cupellation. 

TIN  if  present  in  the  button,  is  quickly  oxidized,  forming 
SnO2,  which,  if  present  in  sufficient  quantity,  covers  the  lead  with 
infusible  yellow  scoria  and  stops  cupellation. 

IRON  gives  brown  or  black  scoria  if  present  in  large  amounts, 
as  do  also  cobalt  and  manganese.  Small  amounts  of  iron  oxide 
dissolve  in  the  litharge  and  stain  the  cupel  dark  red. 

NICKEL  in  small  quantities  gives  dark  green  scoria  and  greenish 
stains.  Larger  amounts  cause  the  button  to  freeze. 

ANTIMONY  is  readily  soluble  in  lead  in  almost  all  proportions, 
and  for  this  reason  the  button  may  contain  a  large  amount  of  it. 
It  comes  off  in  the  first  stages  of  cupellation,  giving  dense  fumes  of 


CUPELLATION  111 

Sb2O3  and  yellow  scoria  of  antimonate  of  lead  around  the  cupel. 
This  scoria  appears  when  4  per  cent  or  more  of  antimony  is  present. 
It  solidifies  almost  as  soon  as  formed  and  expands  in  so  doing. 
If  much  antimony  is  present  the  cupel  will  be  split  open  by  this 
action,  allowing  the  lead  to  run  out  into  the  muffle.  If  present 
in  smaller  amounts  it  may  simply  crack  the  cupel  and  leave  a 
characteristic  ridge  of  yellow  scoria. 

ARSENIC  acts  much  like  antimony  but  is  not  so  often  carried 
into  the  lead  button.  The  scoria  from  arsenical  lead  is  light 
yellow  and  the  fumes  are  less  noticeable. 

BISMUTH  is  less  readily  oxidized  than  lead  and  thus  tends  to 
remain  with  the  silver  until  most  of  the  lead  has  gone.  It  is 
finally  oxidized  and  absorbed  by  the  cupel  and  leaves  a  ring  of 
orange-yellow  around  the  silver  bead.  For  purposes  of  compar- 
ison it  should  be  noted  that  pure  lead  gives  a  brown-yellow  cupel. 
Bismuth  is  the  only  other  metal  which  behaves  like  lead  in  cupel- 
ing; this,  however,  makes  it  possible  to  cupel  argentiferous  bis- 
muth directly. 

COPPER,  like  bismuth,  is  less  readily  oxidized  than  lead,  but 
it  differs  from  bismuth  in  that  its  oxide  alone  is  not  liquid  at  the 
temperature  of  cupellation.  Cuprous  oxide,  however,  is  readily 
soluble  in  molten  litharge  and  the  mixed  oxides  are  absorbed  in 
the  cupel,  giving  a  stain  which  ranges  from  dirty  green  almost  to 
black  according  to  the  amount  present.  The  intensity  of  the 
green  coloration  may  be  taken  as  an  indication  of  the  amount 
of  copper  present  in  the  button.  Even  very  small  amounts  may 
be  detected  in  this  way.  Owing  to  the  relative  difficulty  with 
which  it  oxidizes,  copper  tends  to  concentrate  in  the  button  and  if 
too  large  in  amount  causes  it  to  freeze.  Sometimes  it  will  go  down 
to  a  small  amount  and  then  flatten  out,  leaving  a  copper-colored 
bead. 

TELLURIUM  gives  the  surface  of  the  cupel  a  pinkish  color  most 
of  which  fades  away  upon  cooling.  If  much  tellurium  is  present 
it  gives  a  frosted  appearance  to  the  bead.  In  an  experiment  with 
200  milligrams  of  silver  and  10  grams  of  lead,  the  frosting  made 
its  appearance  when  40  milligrams  of  tellurium  were  added. 
Tellurium  reduces  the  surface  tension  of  the  lead  alloy  and  thus 
increases  the  loss  of  the  precious  metals,  and,  as  it  is  less  readily 
oxidized  than  lead,  most  of  it  remains  with  the  precious  metals 
until  a  large  part  of  the  lead  is  removed. 


112  A   TEXTBOOK  OF  FIRE  ASSAYING 

In  attempting  to  determine  the  presence  of  various  elements 
in  the  lead  by  the  color  of  the  cupel  it  must  be  remembered  that 
one  constituent,  particularly  if  it  has  an  intense  coloring  power 
and  more  particularly  if  the  color  produced  is  dark,  will  tend  to 
mask  other  constituents  producing  lighter  and  less  intense  colors. 

Indications  of  Rare  Metals.  —  The  members  of  the  platinum 
group  of  metals  practically  all  yield  evidence  of  their  presence 
either  in  the  appearance  of  the  surface  of  the  finished  bead  or,  in 
some  cases,  during  the  later  stages  of  the  cupellation  process.  We 
are  indebted  mostly  to  Lodge*  and  to  Bannister  f  for  information 
relative  to  the  appearance  of  the  surface  of  the  finished  bead. 
Lodge  worked  with  relatively  large  amounts  of  rare  metals  and 
small  amounts  of  silver,  and  depended  on  the  unaided  eye  for  his 
observations.  Bannister  worked  with  very  much  smaller  quan- 
tities of  rare  metals  and  large  amounts  of  silver  and  gold  and  used 
a  low-power  microscope.  These  indications,  which  should  be 
recognized  by  all  skilful  assayers,  may  be  made  an  important 
contribution  to  our  knowledge  of  the  qualitative  analysis  of  these 
metals.  At  least  they  should  serve  to  put  the  assayer  on  his 
guard  and  cause  him  to  suspect  the  presence  of  rare  metals  in 
samples  of  ores  and  bullion  submitted  without  request  for  their 
determination.  In  general,  where  the  presence  ,of  any  of  these 
metals  is  suspected,  the  cupels  should  be  finished  at  a  reasonably 
high  temperature  and  the  beads  allowed  to  cool  slowly  in  order 
to  fully  develop  their  crystalline  structure. 

PLATINUM.  —  As  little  as  1.6  per  cent  of  platinum  gives  a  char- 
acteristic frosted  appearance  to  a  silver  bead  which  is  visible  to 
the  naked  eye.  Under  a  low-power  microscope  as  little  as  0.4 
per  cent  may  be  detected  in  beads  weighing  only  0.1  gram.  The 
effect  of  platinum  on  gold  beads  is  not  so  marked  as  in  the  case  of 
silver.  The  presence  of  8  per  cent  of  platinum  seems  to  give  a 
maximum  amount  of  roughness  and  frosting  to  the  silver  bead. 
Buttons  which  contain  a  large  amount  of  platinum  flatten  out 
when  near  the  finishing  point  and  refuse  to  drive,  leaving  a  gray, 
mossy-appearing  bead  which  sticks  to  the  cupel.  Such  beads 
usually  retain  considerable  lead.  The  only  other  metal  of  this 
group  which  gives  a  structure  approaching  that  of  platinum  is 
palladium,  the  effect  of  which  is  dealt  with  later. 

*  Notes  on  Assaying,  1907. 

t  Trans.  Inst.  Min.  Met.  23,  pp.  163-173. 


CUPELLATION  113 

IRIDIUM.  —  Iridium  is  but  slightly  soluble  in  silver  or  gold  at 
the  temperature  of  cupellation,  and  most  of  it  sinks  to  the  bot- 
tom of  the  bead,  where  it  will  appear  as  black  specks.  These 
specks  are  more  readily  distinguished  after  the  bead  has  been  rolled 
into  a  cornet  for  parting.  On  the  surface,  crystal  boundaries 
are  clearly  visible,  the  roughness  being,  according  to  Lodge,  of 
finer  texture  than  that  produced  by  platinum.  Bannister  notes 
that  beads  containing  iridium  were  more  nearly  spherical  than 
normal  beads.  Under  the  microscope,  the"  crystal  faces  were 
strongly  marked  with  lines,  crossing  one  another  after  the  manner 
of  slip-bands.  This  strained  appearance  seemed  to  be  caused 
by  internal  stresses. 

RHODIUM.  —  The  presence  of  mere  traces  of  rhodium  may  be 
detected.  As  little  as  0.004  per  cent  in  silver  beads  was  found  to 
cause  a  distinct  crystallization,  visible  to  the  naked  eye.  The 
facets  of  the  crystals  give  the  appearance  of  a  cut  gem. 

With  0.01  per  cent  of  rhodium  this  appearance  is  more  distinct. 
The  presence  of  0.03  per  cent  of  rhodium  in  silver  causes  the  bead 
to  sprout  and  spit  in  spite  of  all  precautions.  With  this  and 
larger  amounts,  the  surface  of  the  bead  assumes  a  bluish-gray 
color. 

RUTHENIUM.  —  The  presence  of  ruthenium  is  always  indicated 
by  a  black,  crystalline  deposit  firmly  attached  to  the  bead,  usually 
on  the  bottom  edge.  This  is  distinctly  visible  to  the  naked  eye 
even  with  as  little  as  0.004  per  cent.  Under  the  microscope  the 
surface  shows  a  distinct  herringbone  structure. 

OSMIUM.  —  Osmium  is  partly  oxidized  and  volatilized  during 
cupellation.  According  to  Lodge,  if  the  osmium  is  not  completely 
volatilized,  small  black  spots  appear  on  the  silver  bead  when  nearly 
finished.  These  flash  off  and  on,  but  finally  disappear  when  the 
bead  brightens.  Bannister  found  no  specific  indications  of  the 
presence  of  osmium  in  his  tests. 

PALLADIUM.  —  According  to  Lodge,  palladium  gives  the  surface 
a  raised  and  embossed  appearance.  According  to  Bannister,  it  is 
much  like  platinum.  Fortunately  its  presence  is  indicated  by  a 
coloration  of  the  solution  in  the  parting  operation. 

Molten  gold  beads  have  a  beautiful  green  color  and  when  pure 
may  be  cooled  considerably  below  the  true  freezing-point  and 
still  remain  liquid.  On  solidification  they  "  flash  "  as  do  silver 
beads  and  in  solidifying  they  emit  an  apple-green  light.  Ac- 


114  A    TEXTBOOK  OF  FIRE  ASSAYING 

cording  to  Reimsdijk,*  copper  promotes  the  surfusion  of  gold. 
He  also  points  out  the  curious  fact  that  gold  fused  on  a  cupel 
without  the  addition  of  lead  is  not  subject  to  surfusion  and  sets 
gradually  without  flashing.  Fulton  f  measured  the  amount  of 
surfusion  of  silver  beads  obtained  from  cupellation  and  found  it 
to  be  as  much  as  77°  C.  He  found  that  beads  weighing  more  than 
750  milligrams  would  not  flash. 

Very  small  quantities  of  iridium,  rhodium,  osmium,  ruthenium 
and  iridosmium  prevent  this  flashing!  in  gold  beads  and  probably 
also  in  silver  beads.  This  gives  us  another  indication  of  the  pres- 
ence of  the  more  uncommon  metals  of  the  platinum  group.  It 
should  be  noted  in  this  connection  that  small  amounts  of  platinum 
and  palladium  do  not  hinder  flashing.  Over  6  per  cent  of  platinum, 
however,  does  prevent  it. 

In  cupeling  pure  gold  or  silver  with  pure  lead  it  is  found  that  the 
part  of  the  cupel  occupied  by  the  bead  as  the  last  of  the  lead  was 
going  off  will  be  stained  green.  The  higher  the  temperature  and 
consequently  the  higher  the  loss  of  precious  metals,  the  larger  this 
green  area  becomes.  Certain  brands  of  patent  cupels  give  a  large 
amount  of  this  green  stain,  and  whenever  this  is  found  a  serious 
loss  of  silver  is  found  to  have  occurred. 

Retention  of  Base  Metals.  —  It  has  already  been  mentioned 
that  a  plus  error  may  be  incurred  because  of  the  retention  of  lead 
in  the  silver  bead.  If  the  bead  contains  much  lead,  it  will  appear 
dull  or  slightly  yellow,  being  thinly  coated  with  litharge;  the 
part  resting  against  the  cupel  will  be  smooth,  and  it  will  not  blick. 
Occasionally  a  bead  will  show  the  play  of  colors  and  even  flash, 
and  still  retain  as  much  as  1  or  2  per  cent  of  lead.  Sprouting, 
however,  is  considered  proof  of  the  absence  of  all  but  traces  of 
impurities.  When  the  alloy  contains  copper,  the  silver  beads 
may  retain  from  1  to  2  per  cent  of  copper  without  showing  any  un- 
usual symptoms. 

All  gold  and  silver  beads  cupeled  in  the  absence-  of  copper  prob- 
ably retain  small  amounts  of  other  impurities,  and  in  beads  cupeled 
in  the  presence  of  copper  there  is  probably  always  some  copper  left. 
Hillebrand  and  Allen§  found  gold  beads  retaining  from  0.30  to 

*  Chem.  News,  41,  pp.  126,  266. 

f  West  Chem.  and  Met.  4,  p.  50. 

J  Reimsdijk,  loc.  cit 

§  Bulletin  No.  253  U.  S.  Geological  Survey. 


CUPELLATION  115 

0.37  per  cent  of  lead.  Keller*  reports  the  results  of  analysis  of 
30  grams  of  beads  resulting  from  scorification  assays.  This 
showed  0.16  per  cent  of  lead  and  0.15  per  cent  of  bismuth,  the 
latter  having  been  concentrated  from  the  granulated  lead  used 
for  scorification,  which  at  the  time  contained  approximately  0.02 
per  cent  of  that  metal.  Another  series  of  tests  by  Kellerj  showed 
total  impurities  averaging  0.45  per  cent.  It  also  appears  from 
Keller's  work  that  large  beads  are  of  slightly  lower  fineness  than 
small  ones. 

The  persistency  with  which  a  small  amount  of  copper  is  re- 
tained by  gold  and  silver  beads  is  taken  advantage  of  in  the 
gold  bullion  assay  to  toughen  the  beads  so  that  they  can  be  rolled 
thin  without  cracking.  A  small  amount  of  lead,  which  is  invari- 
ably retained  in  the  absence  of  copper,  causes  the  fillet  to  crack. 
Copper  does  not  have  this  effect.  It  is,  therefore,  customary  to 
add  copper  when  none  is  present  in  the  bullion.  Being  less  readily 
oxidized  than  lead,  it  serves  as  an  oxygen  carrier  and  permits 
the  entire  elimination  of  lead  from  the  beacl. 

These  retained  metals  tend  to  compensate  for  the  cupellation 
losses  but  do  not  do  so  entirely,  so  that  ordinary  assay  results  are 
still  at  least  1  or  2  per  cent  low.  Experiments  with  fine  silver, 
however,  reveal  the  interesting  fact  that  by  assaying  slags  and 
cupels  and  adding  the  metal  recovered  to  the  weight  of  the  original 
bead,  a  very  close  check  on  the  original  silver  is  obtained,  provided 
the  assay  was  carefully  made  and  cupeled  with  crystals.  This 
shows  that  the  impurities  in  the  assay  beads  balance  the  volatil- 
ization and  other  miscellaneous  losses.  This  is  only  true,  however, 
when  cupellations  and  scorifications  are  run  at  a  low  temperature. 
Keller^  gives  figures  which  show  that  if  scorifications  and  cupel- 
lations are  run  hot  a  decided  volatilization  loss  occurs  and  cor- 
rected figures  are  still  far  too  low. 

Portland  Cement  and  Magnesia  Cupels.  —  Cupels  of  Portland 
cement  and  calcined  magnesia  have  found  favor  in  some  localities, 
the  former  mostly  in  the  United  States  and  Canada,  the  latter 
principally  in  England  and  South  Africa.  Portland  cement 
cupels  are  made  from  neat  cement  with  from  6  to  10  per  cent  of 
water,  in  the  usual  way.  If  properly  made  and  handled,  they  do 

*  Trans.  A.I.M.E.  60,  p.  706. 

t  Trans.  A.I.M.E.  46,  p.  783  (1913). 

J  Trans.  A.I.M.E.  60,  p.  706. 


116  A   TEXTBOOK  OF  FIRE  ASSAYING 

not  crack,  and  they  absorb  nearly  their  own  weight  of  litharge. 
The  silver  loss  due  to  absorption  is  greater  than  for  bone-ash. 

J.  W.  Merritt*  compared  the  results  obtained  with  bone-ash 
and  cement  cupels  and  found  two  essential  points  of  difference: 
1st,  Beads  from  a  bone-ash  cupel  were  well  rounded  and  stood  on 
a  small  base,  while  those  from  Portland  cement  cupels  were  natter 
and  stood  on  a  base  as  wide  as  the  broadest  diameter  of  the  bead; 
2nd,  The  cement  sticks  tenaciously  to  the  beads  from  Portland 
cement  cupels  but  hardly  at  all  to  those  from  bone-ash  cupels. 

Mann  and  Clay  ton,  t  in  a  study  of  cupellation  losses,  found 
cement  cupels  to  give  very  high  silver  losses  even  under  the  most 
favorable  conditions.  They  also  found  it  very  hard  to  clean  the 
bottom  of  the  bead  without  danger  of  loss. 

Cement  cupels  are  very  much  cheaper  and  more  durable  than 
bone-ash,  but  on  account  of  the  above  disadvantages  should 
not  be  used  for  careful,  uncorrected  silver  assays.  One  disadvan- 
tage of  cement  cupels  for  gold  assays  is  the  extra  care  which  must 
be  taken  in  cleaning  the  bead.  This  is  necessary  because,  in  part- 
ing, a  considerable  amount  of  the  cement  would  remain  insoluble 
as  gelatinous  silica  and  would  be  weighed  as  gold.  Bone-ash, 
on  the  other  hand,  is  practically  entirely  dissolved. 

Magnesia  cupels  are  very  hard,  which  is  an  advantage  in  that 
they  do  not  suffer  so  much  breakage  in  shipment.  They  are 
always  factory-made  and  are  decidedly  more  expensive  than 
bone-ash  cupels,  which  may  be  home-made.  Certain  brands  of 
magnesia  cupels  give  an  apparently  lower  loss  of  silver  in  cupeling 
than  can  be  obtained  with  bone-ash  cupels  but  it  is  a  question  how 
much  of  this  is  real  and  how  much  due  to  an  increase  in  the  amount 
of  impurities  retained  in  the  silver  beads. 

Magnesia  cupels  behave  quite  differently  from  ordinary  bone- 
ash  cupels,  and  the  assayer  who  is  accustomed  to  bone-ash  cupels 
will  have  to  learn  cupeling  over  again  when  he  starts  using  those 
made  of  magnesite.  This  difference  in  behavior  is  due  mainly 
to  the  different  thermal  properties  of  the  two  materials.  Both 
the  specific  heat  and  the  conductivity  of  magnesite  are  decidedly 
greater  than  those  of  bone-ash,  so  that  with  cupels  of  both  kinds 
running  side  by  side,  the  lead  on  the  magnesia  cupel  is  compara- 
tively dull  while  that  on  bone-ash  is  very  bright.  This  is  due  to 

*  Min.  and  Sci.  Press.  100,  p.  649. 

t  Technical  Bulletin,  Vol.  II,  No.  3,  Missouri  School  of  Mines,  p.  33. 


CUPELLATION  117 

the  greater  conductivity  of  magnesite,  which  allows  a  more  rapid 
dispersion  of  the  heat  of  oxidation  of  the  lead,  with  the  result  that 
magnesia  cupels  require  a  higher  muffle  temperature  than  do 
bone-ash  cupels.  An  especially  high  finishing  temperature  is 
required  for  magnesite  cupels,  to  insure  the  elimination  of  the  last 
1  or  2  per  cent  of  lead.  A  bone-ash  cupel  will  finish  in  a  muffle, 
the  temperature  of  which  is  sufficient  to  cause  uncovering,  but  this 
is  not  true  of  the  magnesia  cupel,  because  in  this  case  the  heat  of 
oxidation  of  the  lead  is  diffused  too  rapidly  and  is  not  conserved 
to  help  out  at  the  finish. 

Magnesia  cupels  absorb  about  two-thirds  of  their  own  weight 
of  litharge,  those  of  cement  about  three-fourths  of  their  weight 
of  litharge. 

Color  Scale  of  Temperature.  —  Starting  from  the  lowest  visible 
red  the  temperatures  of  incandescent  bodies  can  be  approximated 
by  the  color  impressions  produced  on  the  eye.  Such  estimates  are, 
of  course,  dependent  upon  individual  judgment  and  the  sus- 
ceptibility of  the  eye,  also  upon  the  amount  of  illumination  of  the 
locality  in  which  the  observation  is  made  and  upon  the  nature  of 
the  heated  body  itself.  The  following  color-temperature  scale* 
will  be  found  convenient  for  reference  in  cupellation. 

Degrees  Centigrade 

Lowest  red  visible  in  the  dark 470 

Dark  red,  blood-red 550 

Dark  cherry 625 

Cherry-red,  full  cherry 700 

Light  red 850 

Orange 900  *^ 

Light  orange 950 

Yellow 1000 

Light  yellow 1050 

White 1150-1200 

*  Howe,  Eng.  and  Min.  Jour.,  69,  p.  75. 


CHAPTER  VI. 
PARTING. 

Parting  is  the  separation  of  silver  from  gold  by  means  of  acid. 
In  gold  assaying  nitric  acid  is  almost  exclusively  used,  although 
sulphuric  acid  is  usually  employed  for  parting  large  lots  of  bullion. 
Nitric  acid  cannot  be  used  successfully  to  separate  silver  from  gold 
unless  there  is  present  at  least  three  times  as  much  silver  as  gold. 
With  this  ratio  the  alloy  must  be  in  a  thin  sheet  and  it  requires  a 
long-continued  heating  with  acid  of  1.26  specific  gravity  to  effect 
a  separation.  In  parting  beads  from  ore  assays  it  is  best  to  have 
at  least  eight  or  ten  times  as  much  silver  as  gold  present,  and  for 
ease  of  manipulation  this  ratio  of  silver  to  gold  is  preferable  to  a 
greater  one.  With  much  less  silver  than  this  a  long-continued 
treatment  with  acid  is  necessary,  while  with  much  more  silver 
than  this,  special  precautions  have  to  be  taken  to  prevent  the  gold 
from  breaking  up  into  small  particles  which  are  difficult  to  man- 
age. The  idea  of  parting  is  to  so  manipulate  that  the  gold  will, 
if  possible,  remain  in  one  piece. 

The  nitric  acid  for  parting  must  be  free  from  hydrochloric  acid 
and  chlorine  in  order  to  have  no  solvent  action  on  the  gold  and 
also  because  any  chlorides  present  would  precipitate  insoluble  silver 
chloride  on  the  gold.  The-  acid  strength  is  of  great  importance 
and  the  proper  strength  to  be  used  depends  upon  the  composition 
of  the  alloy.  The  higher  the  ratio  of  silver  in  the  alloy,  the  less 
the  acid  strength  should  be. 

Great  care  is  necessary  in  parting  to  avoid  breaking  up  the  gold 
and  subsequently  losing  some  of  the  small  particles,  as  well  as  to 
insure  complete  solution  of  the  silver. 

Different  authorities  recommend  different  vessels  for  parting; 
but  for  ore  assays,  and  especially  for  beginners  in  the  art,  the  use 
of  a  porcelain  crucible  or  capsule  is  recommended  and  will  be 
described  first.  Parting  in  flasks  or  test-tubes  with  the  use  of  an- 
nealing cups  will  also  be  discussed  so  that  either  method  may  be 
used. 

118 


PARTING  119 

Parting  in  Porcelain  Capsules.  —  A  glazed  porcelain  capsule 
If  inches  in  diameter  and  1  inch  high  is  preferable  for  this  work  on 
account  of  its  broad  flat  base,  but  a  small  porcelain  crucible  does 
very  well  if  care  is  taken  not  to  upset  it.  Many  different  strengths 
of  acid  and  other  details  of  manipulation  have  been  recommended, 
but  the  procedure  given  below  is  one  which  has  given  uniformly 
satisfactory  results  to  the  author  in  his  laboratory.  The  strength 
of  acid  which  may  be  used  depends  on  the  proportion  of  gold  and 
silver  in  the  alloy;  the  less  the  ratio  of  silver  to  gold,  the  stronger 
the  acid  may  be  without  danger  of  breaking  up  the  gold.  It  is 
not  necessary  that  the  method  to  be  described  should  be  followed 
in  every  case,  but  this  method  is  a  safe  one  for  the  treatment  of 
beads  having  almost  any  proportion  of  silver  to  gold,  from  3  to 
1000  or  more  parts  of  silver  to  1  of  gold. 

PROCEDURE.  —  Pour  into  the  capsule  about  half  an  inch  of 
dilute  nitric  acid  of  1.06  sp.  gr.,  made  by  diluting  1.42  acid  with 
seven  times  its  volume  of  water.  Put  on  the  hot-plate  and  heat 
until  vapor  can  be  seen  rising  from  it,  and  then  drop  in  the  bead 
which  should  be  f  ee  from  adhering  bone-ash.  In  case  the  alloy  has 
only  3  or  4  parts  of  silver  to  1  of  gold  it  must  be  hammered  or 
rolled  out  to  the  thickness  of  an  ordinary  visiting  card,  say  to  0.01 
inch.  The  bead  should  begin  to  dissolve  at  once,  giving  off  bubbles 
of  nitrogen  oxides.  If  it  does  not  begin  to  dissolve,  add  nitric 
acid,  1.26  sp.  gr.,  a  few  drops  at  a  time  until  action  starts.  The 
solution  should  be  kept  hot  but  not  boiling.  The  action 
should  be  of  moderate  intensity.  Continue  the  heating  until 
action  ceases  and  then  decant  the  solution  into  a  clean  white 
evaporating  dish  in  a  good  light,  taking  care  not  to  pour  off  any 
of  the  gold.  Then  add  a  few  cubic  centimeters  of  1.26  sp.  gr. 
acid,  made  by  diluting  strong  nitric  acid,  1.42  sp.  gr.,  with  an 
equal  volume  of  water,  and  heat  almost  to  boiling  for  from  two  to 
ten  minutes.  Decant  this  solution  and  then  wash  three  times  with 
warm  distilled  water,  decanting  as  completely  as  possible  after 
each  washing.  Apply  the  stream  of  water  from  the  wash  bottle 
tangentially  to  the  sides  of  the  capsule,  rotating  it  meanwhile  to 
prevent  direct  impact  of  the  stream  on  the  gold.  After  the  final 
washing  manipulate  the  particles  of  gold  so  as  to  bring  them  to- 
gether, decant  off  the  last  drops  of  water  as  completely  as  possible 
and  set  the  cup  on  a  warm  plate  to  dry  the  gold,  but  avoid  too 
high  a  temperature  as  the  sputtering  of  the  last  drop  of  water 


120  A   TEXTBOOK  OF  FIRE  ASSAYING 

would  tend  to  break  up  and  possibly  throw  out  the  gold.  Finally 
"  anneal  "  the  gold  by  putting  the  cup  in  the  muffle  or  over  the 
open  flame  until  the  bottom  is  bright  red,  when  the  gold  will 
change  from  its  black  amorphous  condition  to  the  true  yellow 
color  of  pure  gold.  It  is  now  ready  to  cool  and  weigh.  To  trans- 
fer the  gold  from  the  cup  to  the  scale-pan,  bring  the  scale-pan  to 
the  front  part  of  the  balance.  Gradually  invert  the  cup  over  the 
pan,  tapping  it  meanwhile  with  a  pencil.  When  this  is  done 
the  gold  will  usually  slide  out  without  difficulty.  If  any  small 
particles  stick  to  the  cup  they  may  be  detached  by  touching  them 
gently  with  the  point  of  the  forceps  or  a  small  camel 's-hair  brush. 

The  gold  should  be  pure  yellow  throughout  and  may  be  com- 
pared with  parted  gold  of  known  purity.  If  it  is  lighter-colored 
than  pure  gold  it  is  probable  that  all  of  the  silver  has  not  been 
dissolved.  If  it  is  dark  in  spots  or  if  the  cup  is  stained,  it  indi- 
cates incomplete  removal  of  the  silver  nitrate.  The  "  anneal- 
ing "  causes  the  gold  to  stick  together,  making  it  easier  to  handle, 
tends  to  burn  out  any  specks  of  organic  matter  which  may  have 
fallen  into  the  cup,  allows  the  assayer  to  observe  the  color  of  the 
parted  gold  and  to  determine  its  purity  in  that  way  and  to  dis- 
tinguish and  separate  any  specks  of  foreign  matter  such  as  fire 
brick,  coke  dust  etc.,  which  may  have  found  their  way  into  the 
cup.  The  "  annealing  "  at  a  red  heat  is  also  necessary  in  order 
that  the  gold  may  contract  and  lose  most  of  its  porosity,  since 
otherwise  it  would  condense  a  considerable  quantity  of  gas  during 
weighing. 

After  the  silver  has  been  dissolved  from  a  dore  alloy  by  the  acid, 
the  gold  remains  as  a  porous  mass  which  is  more  compact  the 
larger  the  proportion  of  gold  the  alloy  contained,  the  thicker  the 
alloy  and  the  less  the  mechanical  disturbance  of  the  bead  during 
solution.  In  treating  a  bead  which  is  near  the  limiting  ratio  of 
silver  to  gold  it  is  sometimes  difficult  to  determine  whether  or 
not  it  is  parted.  This  may  be  ascertained  by  touching  it  with  a 
glass  rod  drawn  down  to  a  rather  small  diameter,  (approximately 
1/32  inch).  If  it  feels  soft  throughout  and  can  be  broken  up  it  is 
practically  parted,  but  it  should  be  heated  almost  to  boiling  with 
1.26  sp.  gr.  acid  for  at  least  ten  minutes  to  ensure  dissolving  the 
last  of  the  silver.  Such  a  mass  of  parted  gold  will  require  a  longer 
and  more  careful  washing,  for  on  account  of  its  density  a  longer 
time  is  required  for  the  silver  nitrate  to  diffuse  through  its  minute 


PARTING  121 

pores.  In  parting  the  ordinary  bead  containing  ten,  twenty  or 
more  times  as  much  silver  as  gold,  it  is  easy  to  see  when  parting 
is  complete  by  the  considerable  shrinking  of  the  mass. 

Notes:  1.  The  nitric  acid  solution  should  be  hot  before  dropping  in 
the  bead  as  in  cold  acid  the  gold  tends  to  break  up  into  extremely  fine  particles. 

2.  The  violent  mechanical  disturbance  due  to  boiling  6r  too  rapid  solu- 
tion may  cause  the  gold  to  break  up,  causing  difficulty  or  actual  loss  in  wash- 
ing and  subsequent  handling. 

3.  If  there  remain  only  a  few  tenths  of  a  milligram  of  porous  gold  the  ten 
minutes  heating  with  1.26  sp.  gr.  acid  is  unnecessary. 

4.  Strong  nitric  acid  (1.46  sp.  gr.)  should  not  be  used  at  any  time,  as 
gold  is  slightly  dissolved  by  it. 

5.  If  in  doubt  at  any  time  as  to  the  purity  of  the  parted  gold,  wrap  it  up 
six  times  its  weight  of  silver  foil  and  carefully  cupel  with  lead,  then  repart 
and  weigh. 

6.  If  a  small  particle  of  gold  is  seen  floating  on  the  surface  of  the  liquid, 
it  may  be  made  to  sink  by  touching  it  with  a  glass  rod. 

7.  The  black  stain  occurring  in  parting  cups    after   heating   is   due   to 
metallic  silver  reduced  from  silver  nitrate  by  the  heat,  showing  insufficient 
washing. 

Inquartation.  —  When  the  bead  contains  too  little  silver  to 
part,  it  is  necessary  to  alloy  it  with  more  silver.  This 'process  is 
called  inquartation.  It  originated  from  the  custom  of  the  old 
assayers  of  adding  silver  until  the  gold  was  one-quarter  of  the 
whole.  They  considered  a  ratio  of  3  parts  of  silver  to  1  of  gold 
to  be  necessary  for  parting.  At  present,  in  assaying  gold  bullion, 
a  ratio  of  only  2  or  2J  parts  of  silver  to  1  of  gold  is  used,  mainly 
to  avoid  all  danger  of  the  gold  breaking  up  in  the  boiling  acid. 
In  this  case  some  little  silver  remains  undissolved,  even  though 
the  alloy  is  rolled  out  to  about  0.01  inch  in  thickness. 

To  inquart  a  bead  wrap  it  with  six  to  ten  times  its  weight  of 
silver  in  4  or  5  grams  of  sheet  lead  and  cupel.  Rose*  considers 
that  different  proportions  of  silver  should  be  used  according  to 
the  weight  of  the  gold,  and  gives  the  following  suitable  proportions : 

Weight  of  Gold  Ratio  of  Silver  to  Gold 

Less  than  0.1  mg., 20  or  30  to  1 

About  0.2  mg., 10  to  1 

About  1.0  mg., 6  to  1 

About  10  mg., 4  to  1 

More  than  50  mg., 2J  to  1 

*  Metallurgy  of  Gold,  Sixth  Ed.,  p.  511. 


122  A   TEXTBOOK  OF  FIRE  ASSAYING 

Many  assayers,  when  working  for  both  gold  and  silver  and  sus- 
pecting an  ore  to  be  deficient  in  silver,  add  silver  to  the  crucible 
or  to  the  lead  button  before  cupeling,  part  directly  and  then  run 
separate  assays  to  determine  the  silver  in  the  ore. 

Preparing  Large  Beads  for  Parting.  —  Large  beads,  especially 
those  which  approach  the  maximum  ratio  of  25  per  cent  gold, 
must  be  flattened  on  an  anvil  and  rolled  out  to  a  thickness  of 
about  0.01  inch  before  parting.  During  this  process  the  alloy 
will  require  frequent  annealing  to  prevent  it  from  cracking.  It 
should  finally  be  rolled  up  into  a  little  "  cornet  "  before  parting. 
(See  "  Assay  of  Gold  Bullion.") 

Parting  in  Flasks,  etc.  —  Parting  in  flasks,  test-tubes,  etc.  is, 
up  to  the  completion  of  the  washing  of  the  gold,  exactly  similar 
to  parting  in  porcelain  capsules.  From  this  point  on,  however, 
the  manipulations  are  different,  as  the  annealing  is  not  done  in 
the  same  vessel,  but  in  an  annealing  cup.  The  annealing  cup  is 
a  small  unglazed  crucible  made  of  fire  clay  and  very  smooth  on 
the  inside. 

PROCEDURE.  —  After  washing  the  gold,  fill  the  flask  or  test-tube 
with  distilled  water,  invert  over  it  an  annealing  cup  and  then 
quickly  invert  the  two  so  that  the  gold  may  fall  into  the  cup. 
This  operation  should  be  done  in  a  good  light  and  preferably 
against  a  white  background.  Tap  the  flask  if  necessary,  to  dis- 
lodge any  gold  which  may  have  caught  on  the  side,  and  after  all 
the  gold  has  settled,  raise  the  flask  slowly  until  its  lip  is  level 
with  the  top  of  the  annealing  cup.  Now,  when  all  the  gold  is 
at  the  bottom  of  the  cup,  slip  the  flask  quickly  from  the  cup  and 
invert  it.  Drain  the  water  from  the  cup,  cover  it  and  set  it  on 
the  hot-plate  to  dry.  When  fully  dry,  it  is  ready  to  be  annealed 
and  weighed.  Examine  the  flask  once  more  to  make  sure  that 
no  gold  has  been  left  in  it. 

This  method  of  parting  has  the  advantage  that  the  acid  may 
be  boiled,  if  necessary,  with  less  danger  of  its  boiling  over  and 
causing  loss  of  fine  gold.  It  is  well  suited  for  the  parting  of  large 
beads  where  the  porcelain  cup  would  not  contain  enough  acid  to 
dissolve  all  of  the  silver,  and  also  to  the  parting  of  alloys  where 
the  ratio  of  silver  to  gold  is  only  2  or  3  to  1,  and  which  therefore 
require  a  long-continued  heating  at  or  near  boiling  temperature. 
The  method  is  therefore  recommended  for  use  in  the  assay  of  gold 
bullion.  The  clay  cups  have  the  advantage  of  porosity  so  that 


PARTING  123 

they  can  absorb  the  last  drops  of  water  and  give  it  off  again 
slowly,  thus  preventing  spattering  if  they  are  set  on  a  hot  iron 
plate  to  dry,  They  also  stand  sudden  changes  of  temperature 
somewhat  better  than  the  glazed  porcelain  cups. 

This  method  has  the  disadvantage  that  if  all  the  parted  gold 
does  not  remain  in  one  piece,  there  is  greater  danger  of  loss,  be- 
cause the  fine  gold  settles  with  difficulty  and  because  it  cannot 
be  watched  so  well  through  all  stages  of  the  process.  There  is 
also  danger  of  small  particles  of  the  cup,  and  especially  the  cover, 
being  broken  off  and  mixed  with  the  gold. 

Influence  of  Base  Metals  on  Parting.  —  Pure  gold-silver  alloys 
of  almost  any  proportions  not  exceeding  30  per  cent  of  gold,  with 
a  proper  strength  of  nitric  acid  and  the  right  degree  of  heat,  will 
part  and  leave  the  gold  in  a  coherent  mass  similar  in  shape  to 
the  original  alloy,  although  very  much  reduced  in  size  when  but 
little  gold  is  present.  While  a  relatively  strong  acid  may  be 
used  directly  on  alloys  containing  large  proportions  of  gold,  this 
same  strength  of  acid  cannot  be  used  on  those  alloys  containing 
but  little  gold,  without  producing  disintegration.  Heating  of 
the  parting  acid  is  more  necessary,  too,  when  but  little  gold  is 
present.  The  fact  that  the  gold  holds  together  under  these 
circumstances,  and  contracts,  following  the  retreating  surface  of 
silver,  very  much  simplifies  the  determination.  When  the  gold 
breaks  up,  it  increases  the  difficulty  for  the  assayer  and  causes 
unavoidable  losses  in  decantation.  The  high  temperature  of  the 
parting  acid  increases  the  mobility  of  the  gold,  and  in  the  case  of 
pure  alloys  prevents  it  from  breaking  up. 

Keller*  states  that,  although  assay  beads  of  not  less  than  997 
parts  gold  plus  silver  fineness  offer  no  difficulty  in  parting,  yet 
when  the  fineness  falls  to  990  or  lower  the  gold  cannot  be  obtained 
in  any  form  other  than  powder,  no  matter  what  acid  and  heat 
combinations  are  employed.  The  explanation  of  this  difference 
may  be  that  the  whole  gold-silver  series  form  solid  solutions, 
in  which  the  molecules  of  gold,  even  when  in  dilute  solution, 
are  uniformly  distributed  and  almost  if  not  quite  in  contact, 
or  at  least  within  spheres  of  mutual  attraction.  It  may  be 
imagined  that  impurities  forming  compounds  or  eutectic  mix- 
tures may  so  disrupt  the  uniformity  of  texture  of  the  alloy,  and 

*  Trans.  A.I.M.E.  60,  p.  706. 


124  A    TEXTBOOK  OF  FIRE  ASSAYING 

therefore  the  continuity  of  the  gold,  as  to  prevent  its  cohesion 
during  the  acid  treatment. 

There  is  naturally  a  gradation  in  the  degree  of  gold  disintegra- 
tion, from  its  almost  complete  cohesion  when  derived  from  prac- 
tically "  fine  "  beads,  to  its  completely  pulverulent  form  when  de- 
rived from  beads  of  990  fineness. 

Indications  of  Presence  of  Rare  Metals.  —  As  has  been  indica- 
ted in  the  chapter  on  cupellation,  the  assay  beads  may  contain, 
in  addition  to  traces  of  lead,  bismuth,  copper  and  tellurium, 
practically  all  of  the  platinum,  palladium,  iridium,  iridosmium, 
as  well  as  more  or  less  of  the  rhodium,  osmium  and  ruthenium  con- 
tained in  the  original  material  which  was  assayed.  Most  of  these 
rare  metals  make  their  presence  known  by  the  appearance  of  the 
bead.  If  they  are  not  discovered  in  the  bead,  indications  of 
their  presence  may  be  found  during  parting. 

In  nitric  acid  parting  a  considerable  part  of  the  platinum, 
palladium  and  osmium  are  dissolved,  the  amount  depending  on 
various  conditions  such  as  the  amount  of  silver  present,  the 
strength  of  acid,  etc. 

According  to  Rawlins,*  PLATINUM  has  a  disintegrating  effect 
upon  the  gold,  when  the  latter  does  not  make  up  more  than  5 
per  cent  of  the  weight  of  the  bead.  As  platinum  is  only  partly 
soluble,  the  remaining  insoluble  platinum  discolors  the  gold, 
leaving  it  steel-gray  instead  of  yellow.  Furthermore,  platinum 
gives  the  parting  acid  a  brown  or  blackish  color  according  to  the 
amount  present,  but  small  amounts  might  not  be  detected  this 
way.  If,  however,  the  appearance  of  the  bead  leads  one  to  sus- 
pect the  presence  of  platinum  the  above  indications  would  help 
to  confirm  its  presence. 

PALLADIUM  yields  an  orange-colored  solution  in  nitric  acid  part- 
ing. This  test  is  very  delicate,  so  that  even  0.05  milligrams  in  a 
small  bead  gives  a  distinct  coloration  to  the  solution. 

IKIDIUM  appears  in  the  parted  gold  as  detached  black  specks 
retaining  their  color  after  annealing. 

Errors  Resulting  from  Parting  Operations.  —  In  addition  to 
platinum,  iridium,  and  other  of  the  rare  metals  which  may  be 
retained  and  weighed  as  gold  there  is  always  a  small  amount  of 
silver  which  persistently  resists  solution.  The  amount  depends 
upon  a  number  of  factors,  chief  of  which  are  the  ratio  of  silver  to 

*  Trans.  Inst.  Min.  and  Met.  23,  p.  177. 


PARTING  125 

gold  in  the  original  bead,  the  strength  of  acid  used  and  the  time 
of  acid  treatment.  Under  ordinary  conditions  this  silver  re- 
tained probably  amounts  to  about  0.05  per  cent  of  the  weight 
of  the  gold. 

When  gold  disintegrates  in  parting  on  account  of  the  presence 
of  impurities  in  the  bead,  part  of  it  is  invariably  lost  in  decanta- 
tion.  This  decanted  gold  is  often  so  finely  divided  as  to  be 
invisible. 

If  the  parting  acid  contains  impurities,  particularly  chlorine 
in  any  form,  some  of  the  gold  is  sure  to  be  dissolved.  Even 
pure  nitric  acid,  if  concentrated  and  boiling,  dissolves  a  small 
amount,  according  to  Rose*  about  0.05  per  cent.  The  silver 
retained  and  the  gold  dissolved  in  pure  acid  produce  errors  so 
small  as  to  be  negligible,  but  the  loss  resulting  from  the  use  of 
impure  acid  and  the  decantation  loss  must  be  carefully  guarded 
against. 

There  are  a  number  of  errors  in  the  determination  of  gold 
which  should  be  obvious  and  which  can  be  either  avoided  or  cor- 
rected. Such  errors  need  not  be  discussed  here. 

Recovery  of  Gold  Lost  in  Decantation.  —  Because  of  the  effect 
of  impurities  in  the  bead  or  for  other  reasons,  some  of  the  gold 
may  disintegrate  in  parting  and  be  lost  in  decantation.  This 
decanted  gold  is  often  so  finely  divided  as  to  be  invisible,  and  is 
therefore  lost  in  ordinary  commercial  work.  It  may  be  readily 
collected,  however,  by  the  precipitation  of  a  small  amount  of 
silver  chloride  which  carries  it  down  in  settling.  The  precipitate 
containing  the  gold  is  then  filtered  off,  dried  and  scorified  with 
lead.  The  button  is  cupeled  and  the  bead  parted. 

Testing  Nitric  Acid  for  Impurities.  CHLORIDES.  —  To  test  for 
the  presence  of  hydrochloric  acid  take  about  10  c.c.  of  acid  and 
pour  a  few  c.c.  of  silver  nitrate  solution  cautiously  down  the 
side  of  the  tube,  so  that  the  two  liquids  do  not  mix.  If  chlorides, 
bromides  or  iodides  are  present,  a  precipitate  in  the  form  of  a 
ring  will  appear  where  the  two  liquids  come  together.  This 
test  is  more  delicate  than  mixing  the  two  solutions.  It  is  not 
necessary  to  try  to  distinguish  between  the  three  haloids,  as,  if 
any  precipitate  is  found,  the  acid  must  be  either  rejected  or 
purified. 

*  Metallurgy  of  Gold,  Sixth  Ed.,  p.  541. 


126  A    TEXTBOOK  OF  FIRE  ASSAYING 

CHLORATES.  —  Nitric  acid  often  contains  chlorine  in  the  form  of 
chloric  acid,  and  this  does  not  give  a  precipitate  with  silver 
nitrate.  The  chloric  acid  must  first  be  decomposed  before  the 
chlorine  will  precipitate  as  silver  chloride.  To  test  nitric  acid 
for  chlorates  take  about  250  c.c.  in  a  beaker  and  add  1  c.c.  of 
silver  nitrate  solution.  In  the  absence  of  any  precipitate  which 
would  indicate  the  presence  of  chlorides,  etc.,  add  about  5  grams 
of  some  metal  to  reduce  the  chloric  salt.  Silver,  zinc  or  copper 
will  do.  Heat  nearly  to  boiling  to  dissolve  the  metal.  If  chloric 
acid  is  present  a  precipitate  of  silver  chloride  will  form.  To 
confirm,  observe  solubility  in  water  and  in  cold  dilute  ammonia, 
also  effect  of  sunlight. 

Testing  Wash  Water,  etc.  —  The  water  used  for  washing  the 
parted  gold  and  for  diluting  the  nitric  acid  should  be  distilled  to 
ensure  its  purity.  Any  chlorides  in  the  wash  water  will  of  course 
precipitate  in  the  parting  vessel  as  silver  chloride  and  if  not 
removed  will  be  weighed  as  gold.  The  wash  water  should  be 
carefully  tested  by  slightly  acidifying  with  nitric  acid  and  then 
pouring  in  silver  nitrate  as  in  testing  the  nitric  acid.  If  distilled 
water  is  not  available  and  water  at  hand  contains  a  small  amount 
of  chlorides  the  proper  combining  proportion  of  silver  nitrate 
may  be  added,  the  whole  well  mixed  and  then  allowed  to  stand 
until  the  silver  chloride  settles  out.  Care  must  be  taken  to  avoid 
adding  an  excess  of  silver  nitrate,  for  obvious  reasons.  If  by 
any  chance  silver  chloride  has  been  precipitated  in  the  parting 
cups,  the  addition  of  a  little  ammonia  will  cause  it  to  dissolve, 
after  which  the  washing  may  be  completed  with  pure  water. 

Testing  Silver  Foil  for  Gold.  —  It  is  never  safe  to  assume  that 
so-called  C.  P.  silver  is  free  from  gold,  until  it  has  been  tested 
and  proven  so.  A  simple  dissolving  of  the  foil  in  nitric  acid  is 
hardly  sufficient  as  small  amounts  of  gold  might  escape  recog- 
nition in  this  way.  A  better  method  is  to  fuse  about  a  gram  into 
a  bead  and  dissolve  this  slowly  in  hot  dilute  nitric  acid.  The 
best  method  is  to  dissolve  about  10  grams  in  nitric  acid,  dilute 
slightly  and  add  a  small  quantity  of  a  solution  of  sodium  chloride. 
This  will  precipitate  some  silver  chloride  which  will  collect  any 
finely  divided  gold.  Allow  this  precipitate  to  settle,  then  filter 
it  off,  dry  and  scorify  with  lead.  Cupel  the  button  and  part 
the  small  bead  obtained.  This  should  give  all  the  gold  in  one 
piece. 


CHAPTER  VII. 
THE  SCORIFICATION  ASSAY. 

The  scorification  assay  is  the  simplest  method  for  the  deter- 
mination of  gold  and  silver  in  ores  and  furnace  products.  It 
consists  simply  of  an  oxidizing  muffle  fusion  of  the  ore  with 
granulated  lead  and  borax-glass.  The  lead  oxide  formed  com- 
bines with  the  silica  of  the  ore  and  also  to  a  certain  extent  dissolves 
the  oxides  of  the  other  metals.  The  only  reagents  used  other 
than  lead  are  borax-glass  and  occasionally  powdered  silica,  which 
aid  in  the  slagging  of  the  basic  oxides. 

The  scorifier  is  a  shallow,  circular  fire-clay  dish  2  or  3  inches 
in  diameter.  The  sizes  most  commonly  used  are  2  J,  2f  and  3 
inches  in  diameter. 

The  amount  of  ore  used  varies  from  0.05  A.  T.  to  0.25  A.  T., 
the  amount  most  commonly  used  being  0.10  A.  T.  With  this  is 
used  from  30  to  70  grams  of  test  lead  and  from  1  to  5  grams  of 
borax-glass,  depending  on  the  amount  of  base  metal  impurities 
present.  Sometimes  powdered  silica  and  occasionally  litharge 
are  also  used.  With  nearly  pure  galena,  or  a  mixture  of  galena 
and  silica,  a  charge  of  30  to  35  grams  of  test  lead  and  1  gram  of 
borax-glass  will  suffice  for  0.10  A.  T.,  of  ore,  but  when  the  ore  con- 
tains nickel,  copper,  cobalt,  arsenic,  antimony,  zinc,  iron,  tin,  etc., 
a  larger  and  larger  amount  of  lead  and  borax-glass  must  be  used 
according  to  the  relative  ease  with  which  the  metals  are  oxidized 
and  the  solubility  of  their  oxides  in  the  slags  formed.  Of  the 
above,  copper  especially  is  very  difficultly  oxidized  and  when  much 
is  present  in  the  ore  the  lead  button  from  the  first  scorification 
will  have  to  be  rescorified  once  or  twice  with  added  lead.  Iron, 
on  the  other  hand,  is  comparatively  readily  oxidized,  and  except 
for  the  necessity  of  adding  an  extra  amount  of  lead  and  thorax- 
glass  to  make  a  fluid  slag  the  ore  is  as  readily  assayed  as  galena. 
Lime,  zinc,  and  antimony  require  especially  large  amounts  of 
borax-glass  to  convert  th*eir  refractory  oxides  into  a  fusible  slag. 

Solubility  of  Metallic  Oxides  in  Litharge.  —  Litharge  although 
a  strong  base,  has  the  power  of  holding  in  igneous  solution  cer- 

127 


128 


A   TEXTBOOK  OF  FIRE  ASSAYING 


tain  quantities  of  other  metallic  oxides.  This  has  an  important 
bearing  on  the  ease  or  difficulty  with  which  various  metals  may 
be  slagged  in  scorification.  According  to  Berthier  and  Percy, 
the  solubilities  of  the  various  metallic  oxides  in  litharge  are  as 
shown  in  the  following  table : 


One  part  of  Cu2O    CuO 

Requires  parts  of  PbO      1.5       1.8 


ZnO    Fe2O3  MnO  SnO2  TiO2 
8         10         10       12        8 


Antimony  trioxide  (Sb2O3)  dissolves  in  litharge  in  all  propor- 
tions. 

Heat  of  Formation  of  Metallic  Oxides.  —  Another  important 
factor  having  to  do  with  the  elimination  of  impurities  by  scorifi- 
cation is  the  relative  heat  of  formation  of  the  various  metallic 
oxides.  In  a  mixture  of  various  metallic  sulphides,  (assuming 
for  a  moment  the  ignition  temperature  to  be  the  same  for  all), 
that  reaction  in  which  is  evolved  the  greatest  amount  of  heat 
would  naturally  proceed  at  the  fastest  rate.  The  heat  of  com- 
bination of  various  metals  each  with  16  grams  of  oxygen  is  shown 
in  the  following  table.  This  basis  is  used  on  the  assumption  that 
the  amount  of  oxygen  is  limited. 


TABLE   XV. 
HEAT  OF  FORMATION  OF  METALLIC  OXIDES. 


Reaction.     Heat  of  comb,  with 

!6gO 

Reaction.     Heat  of 

comb,  with 

16gO 

Zinc  to  ZnO 
Tin  to  SnO2 

Iron  to  FeO 
Cobalt  to  CoO 
Nickel  to  NiO 

Antimony   to   Sb2O3 

Arsenic  to  As2O3     = 
Lead  to  PbO 

141,300 

84,800 
=  70,650 

65,700 
64,100 
61,500 
KK  fiftn 

Bismuth    to    Bi2O 
Copper  to  Cu2O 
Sulphur  to  SO2 

Tellurium  to  TeO2 
Silver  to  Ag2O 

139,200 

=  46,400 
43,800 

f)A   aoft 

3 
69,260 

2 

166,900 

2 
38,600 

1  Q  onrj 

2 
11,500 

7,000 

3 
156,400 

=  52,100 
50,800 

3 

LrOlQ  tO  AU2Ua         — 

3 

—  3,800 

Ignition  Temperature  of  Metallic  Sulphides.  —  The  ignition 
temperature  of  the  metallic  sulphides  may  also  be  of  interest 
in  this  connection. 


THE  SCORIFICATION  ASSAY 


129 


TABLE  XVI. 
IGNITION  TEMPERATURES1  OF  METALLIC  SULPHIDES   WHEN  HEAPED  IN  AIR. 


Material     . 

Formula 

Ignition 
Temp.0  C. 

Material 

Formula 

Ignition 
Temp.0  C. 

Stibnite 

SbtSa 

290-340 

Galena  2 

PbS 

554-847 

Pyrite 

FeS2 

325-427 

Millerite 

NiS 

573-616 

Pyrrhotite 

FexSx+i 

430-590 

Argentite 

Ag2S 

605-873 

Chalcocite 

Cu2S 

430-679 

Sphalerite 

ZnS 

647-810 

1  Friedrich,  Metallurgie,  6,  p.  170  (1909). 

2  In  Oxygen. 

The  metals  in  Table  XV  are  arranged  in  order  of  their  heats 
of  combination  with  oxygen,  expressed  in  terms  of  a  unit  weight 
of  oxygen.  This  is  the  order  in  which  they  will  be  removed  in 
scorification  or  cupellation.  In  general  it  may  be  said  that  the 
metals  in  a  molten  alloy,  such  as  a  lead  button  on  a  cupel,  or  the 
lead  alloy  in  a  scorifier,  are  oxidized  in  succession,  each  partly 
protecting  those  which  are  less  easily  oxidized  than  itself.  This 
separation  is  not  quantitative  however,  owing  to  the  effect  of 
mass  action,  and  in  the  case  under  discussion,  where  we  have  a 
large  amount  of  lead  and  small  amounts  of  other  metals,  a  con- 
siderable amount  of  lead  would  be  oxidized  during  the  complete 
oxidation  of  the  metals  above  it  in  the  table. 

Those  metals  which  lie  below  lead  in  the  table  will  be  but 
slowly  slagged  in  scorification  and  only  at  the  expense  of  a  large 
amount  of  lead.  Thus,  during  scorification  or  cupellation,  bis- 
muth, copper  and  tellurium  are  concentrated  in  the  residual 
unoxidized  lead  and  this  explains  why  it  is  so  difficult  to  separate 
these  metals  from  silver  by  scorification  and  cupellation.  This 
can  only  be  done  by  repeated  scorification  with  fresh  lead  and,  as 
might  be  expected,  this  will  result  in  a  considerable  loss  of  silver. 

From  their  positions  in  the  table,  it  is  evident  that  silver  is 
not  easily  oxidized  and  that  gold  is  protected  by  all  other  metals. 

To  one  who  has  a  knowledge  of  the  mineral  character  of  the 
ore,  a  glance  at  the  ignition  temperature  of  the  sulphides  shown 
in  Table  XVI  will  afford  an  idea  of  the  initial  temperature  re- 
quired for  scorification.  From  a  comparison  of  the  heats  of 
oxidation  of  the  metals  present  with  that  of  lead,  the  relative 


130  A    TEXTBOOK  OF  FIRE  ASSAYING 

ease  or  difficulty  of  their  elimination  in  scorification  may  be  deter- 
mined; and  a  knowledge  of  the  solubility  of  the  metal  oxides  in 
litharge  and  in  borax  will  indicate  the  relation  between  ore,  lead 
and  flux  which  will  give  the  most  satisfactory  results. 

A  small  amount  of  nickel  will  cause  more  trouble  in  scorifica- 
tion and  cupellation  than  any  other  metal.  This  is  largely  due 
to  the  insolubility  of  its  oxide  in  litharge.  The  presence  of  cop- 
per seems  to  increase  the  difficulty  of  eliminating  it  from  the 
lead.  In  scorifying  ores  containing  nickel  but  no  copper,  prac- 
tically all  of  the  nickel  comes  out  in  the  first  slag,  making  it 
lumpy,  to  be  sure,  but  leaving  the  lead  practically  clean.  When 
copper  is  also  present  the  button  will  require  rescorification  and 
much  nickel  scoria  is  found  in  the  second  scorifier.  Brown* 
calls  attention  to  a  similar  action  of  nickel  in  converting  copper- 
nickel  matte.  In  this  case  the  nickel  does  not  behave  like  iron,  as 
it  might  be  expected  to  do  by  reason  of  its  heat  of  oxidation,  but 
like  copper,  which  has  a  much  lower  heat  of  oxidation.  It  was 
found  impossible  to  slag  off  the  nickel  without  at  the  same  time 
removing  a  large  part  of  the  copper.  In  fact  the  nickel-copper 
alloy  acts  like  one  metal  and  follows  the  same  laws  that  govern 
the  behavior  of  copper  alone.  No  adequate  explanation  of  this 
behavior  is  known. 

SCORIFICATION  ASSAY   OF   SILVER   ORE. 

Procedure.  —  Empty  the  bottle  or  envelope  of  ore  on  to  a 
sheet  of  glazed  paper  or  oilcloth  and  mix  thoroughly  by  rolling. 

Take  three  scorifiers,  2J,  2|,  and  3  inches  in  diameter  respec- 
tively. Weigh  out  on  the  flux  balance  three  portions  of  granu- 
lated lead  35,  45,  and  55  grams  respectively.  Divide  each  lot  of 
lead  approximately  in  halves,  transfer  one-half  of  each  to  the 
corresponding  scorifier  and  reserve  the  remaining  portions. 
Weigh  out  three  portions  of  exactly  0.1  A.  T.  of  ore  on  the  pulp 
balance  and  place  on  top  of  the  lead  in  the  scorifiers.  Mix  thor- 
oughly with  the  spatula  and  cover  with  the  remaining  portions 
of  lead.  Scatter  1  or  2  grams  of  borax-glass  on  top  of  the  lead. 
The  scorifiers  are  now  ready  for  the  muffle,  ,  which  should  be 
light  red  or  yellow  before  the  charges  are  put  in.  This  tempera- 
ture should  be  maintained  during  the  first  part  of  the  roasting 
period. 

*  Trans.  A.I.M.E.,  41,  p.  296. 


THE  SCORIFICATION  ASSAY  131 

FUSION  PERIOD.  —  Place  the  scorifiers  about  midway  in  the 
muffle,  close  the  door  and  allow  the  contents  to  become  thor- 
oughly fused. 

ROASTING  PERIOD.  —  When  thoroughly  fused,  open  the  door 
to  admit  air  to  oxidize  the  ore  and  lead.  If  the  ore  contains 
sulphides  these  will  now  be  seen  floating  on  the  top  of  the  molten 
lead.  The  sulphur  from  these  is  burned  going  off  as  SO2  and  the 
base  metals  are  oxidized  and  slagged.  The  precious  metals 
remain  unoxidized  and  are  taken  up  by  the  lead  bath.  These 
patches  of  ore  grow  smaller  and  soon  disappear,  after  which  the 
surface  of  the  melt  becomes  smooth,  consisting  of  a  bath  of  mol- 
ten lead  surrounded  by  a  ring  of  slag. 

The  vapor  rising  from  the  assays  will  often  indicate  the  char- 
acter of  the  ore.  Sulphur  gives  clear  gray  fumes,  arsenic  grayish- 
white  and  antimony  reddish.  Zinc  vapor  is  blackish  and  the 
zinc  itself  may  be  seen  burning  with  a  bright  white  flame. 

SCORIFICATION  PERIOD.  —  The  lead  continues  to  oxidize  and 
the  ring  of  slag  around  the  circumference  of  the  scorifier  be- 
comes larger  as  more  of  the  lead  is  oxidized.  Finally  the  whole 
of  the  lead  is  covered  with  slag  and  the  scorification  is  finished. 
The  ore  should  be  completely  decomposed  and  practically  all 
of  the  gold  and  silver  should  be  alloyed  with  the  metallic  lead. 

LIQUEFACTION  PERIOD.  —  Close  the  door  of  the  muffle  and 
increase  the  heat  for  a  few  minutes  to  make  the  slag  thoroughly 
liquid  and  to  ensure  a  clean  pour.  Then  pour  the  contents  of 
the  scorifiers  into  a  dry,  warm,  scorifier  mold  which  has  been 
previously  coated  with  chalk  or  iron  oxide.  Pour  into  the  cen- 
ter of  the  mold,  being  careful  to  see  that  the  lead  does  not  spatter 
and  that  all  of  it  comes  together  in  one  piece.  The  inside  sur- 
face of  the  scorifiers  should  be  smooth  and  glassy,  showing  no 
lumps  of  ore  or  undecomposed  material. 

When  the  slag  is  cold  examine  it  and  the  sides  of  the  mold 
carefully  for  shots  of  lead.  These  are  most  likely  to  occur  at  the 
contact  of  the  slag  with  the  mold,  and  if  found  should  be  saved 
and  added  to  the  main  button.  Next  separate  the  main  lead 
button  from  the  slag,  hammer  it  into  the  form  of  a  cube  and  weigh 
to  the  nearest  gram  on  the  flux  balance. 

If  the  lead  is  soft  and  malleable,  and  the  color  of  the  scorifier 
does  not  indicate  the  presence  of  large  amounts  of  copper,  nickel 
or  cobalt,  the  button  is  ready  for  cupellation.  If  it  is  hard  or 


132  A    TEXTBOOK  OF  FIRE  ASSAYING 

brittle  it  may  contain  impurities  which  must  be  removed  by  re- 
scorifying  with  an  additional  amount  of  granulated  lead. 

Finally  cupel  and  weigh  the  resultant  silver  or  dore  beads. 
Report  in  your  notes  the  weight  of  ore  and  reagents  used, 
the  weight  of  lead  button  obtained,  as  well  as  the  weight 
and  assay  in  ounces  per  ton  of  gold  and  silver.  Note  also  the 
time  of  scorification  and  cupellation  and  describe  the  appear- 
ance of  the  scorifier  and  cupel. 

Notes:  1.  The  ore  must  be  so  fine  that  a  sample  of  0.1  A.  T.  will  truly 
represent  the  whole;  100-mesh  may  be  fine  enough  for  some  ores,  170-mesh 
may  be  necessary  for  some  others. 

2.  In  weighing  out  the  ore,  spread  the  sample  which  has  been  thoroughly 
mixed,  into  a  thin  sheet  on  the  glazed  paper  at  one  side  of  the  pulp  balance. 
Place  the  weight  on  the  right-hand  pan  and  the  ore  on  the  left-hand  pan. 
With  the  spatula  mark  the  ore  off  into  squares  1  inch  or  so  on  a  side,  and  then 
take  a  small  portion  from  every  square  for  the  sample,  being  sure  to  take  a 
section  from  top  to  bottom  of  the  ore.     During  this  first  sampling  the  scale- 
pan  should  be  held  over  the  paper  in  one  hand  and  the  spatula  in  the  other. 
When  what  is  judged  to  be  the  right  amount  of  ore  is  obtained  the  pan  is  put 
back  oh  the  balance  and  the  hand  with  which  it  was  held  is  used  to  turn  the 
balance  key. 

The  balance  should  be  turned  out  of  action  each  time  ore  is  put  on  or  taken 
off  the  scale-pan  and  the  pointer  need  move  only  1  or  2  divisions  to  indicate 
whether  too  much  or  too  little  ore  is  on  the  pan.  To  obtain  the  final  balance, 
have  a  little  too  much  ore  in  the  pan.  take  off  enough  on  the  point  of  the 
spatula  to  reverse  the  condition  of  balance.  With  the  balance  key  lift  the 
beam  only  enough  to  allow  the  pointer  to  swing  1  or  2  divisions  to  the  left 
of  the  center  and  then  hold  the  key  in  this  position.  Hold  the  spatula  over 
the  pan  and  by  tapping  it  gently  with  the  first  finger  allow  the  ore  to  slide 
off  onto  the  scale-pan  a  few  grains  at  a  time,  until  the  balance  is  restored  and 
the  needle  swings  over  to  the  center.  By  repeating  this  process,  rejecting 
the  ore  retained  on  the  spatula  each  time,  an  exact  weight  can  soon  be 
obtained. 

3.  The  value  of  the  results  depends   upon   the   care  which  is  taken  in 
mixing,  sampling  and  weighing  out  the  charges.     Do  not  attempt  to  save 
time  by  slighting  the  mixing,  for  if  a  true  sample  is  not  obtained  at  this  point 
no  amount  of  subsequent  care  will  avail  to  give  reliable  results. 

4.  Instead    of    being  weighed,  the    granulated    lead    may    be    measured 
with  sufficient  accuracy  by  the  use  of  a  shot  measure  or  small  crucible.     The 
borax-glass  may  also  be  measured. 

5.  The  size  of  scorifier  to  be  used   depends  upon   the  amount   of  ore, 
lead,  borax-glass  and  silica  used,  and  should  be  such  as  to  give  a  button  of 
approximately  15  to  18  grams.      If  a  large  scorifier  is  used  with  a  small 
amount  of  lead  the  resulting  lead  button  will  be  very  small  and  a  high  loss  of 
silver  will  result.     Again,  the  larger  the  amount  of  borax-glass  that  is  used 
the  more  slag  there  will  be  and  the  sooner  the  lead  will  be  covered. 


THE  SCORIFICATION  ASSAY  133 

6.  If  the  contents  of  the  scorifiers  do  not  become  thoroughly  liquid  and 
show  a  smooth  surface  of  slag  after  ten  or  fifteen  minutes,  the  assays  require 
either  more  heat,  more  borax-glass  or  more  lead. 

7.  If  the  ore  contains  much  tin,  antimony,  arsenic,  nickel  or  large  amounts 
of  basic  oxides  such  as  hematite,  magnetite,   etc.,   an  infusible   scoria  is 
almost  certain  to  form  on  the  surface  of  the  slag  or  on  the  sides  of  the  scori- 
fier  which  neither  a  high  temperature   nor  extra   borax-glass   will  remove. 
As  this  scoria  is  likely  to  enclose  particles  of  undecomposed  ore  the  only 
safe  procedure  is  to  make  a  fresh  assay  with  less  ore,  and  with  such  other 
changes  in  charge  and  manipulation  as  the  experience  of  the  first  assay  may 
suggest. 

8.  Ores  containing    pyrite    require    a    higher    temperature    during    the 
roasting  period  than  those  containing  galena. 

9.  Some    assayers   add   litharge    to    the   scorification    charge,  especially 
with  pyritic  ores.     On  heating,  the  litharge  is  reduced  to  metallic  lead,  the 
sulphur  of  the  pyrite  being  oxidized. 

10.  Litharge,  being  a  strong  base,  has  a  great  affinity  for  the  silica  of 
the  scorifier  and,  especially  when  mixed  with  copper  oxide,  it  attacks  this 
silica  readily.     When  scorifying  matte  and  copper  bullion  it  is  often  necessary 
to  add  powdered  silica  to  the  charge  to  prevent  a  hole  being  eaten  through 
the  scorifier. 

11.  The  lead  button  should  weigh  from  12  to  20  grams.     If  it  is  much 
smaller  than  this  there  is  danger  of  a  loss  of  silver  due  to  oxidation,  especially 
when  the  ore  is  rich.     If  the  button  is  too  large  it  may  be  rescbrified  in  a  new 
scorifier  to  the  size  desired. 

12.  Hard  buttons  may   be  due  to  copper,  antimony   or  in  fact  almost 
any  metal  alloyed  with  the  lead.     Brittle  buttons  may  be  due  to  one  of  many 
alloyed  metals,  or  to  the  presence  of  sulphur  or  lead  oxide. 

13.  The  scorifier  slag    should    be    homogeneous    and    glassy.      If   non- 
homogeneous  it  probably  contains  undecomposed  ore. 

14.  The  white  patches  occasionally    found    in    the    slag   are   made  up 
mostly  of  lead  sulphate  which  is  formed  when  the  scorification  temperature 
is  low. 

15.  Scorifier  slags  are   essentially    oxide   slags   and   consist   of   metallic 
oxides  dissolved  in  an  excess  of  molten  litharge,  together  with  smaller  amounts 
of  dissolved  silicates  and  borates. 

16.  If  too  low  an  initial  temperature  is  employed  or  if  the  muffle  door 
is  opened  too  soon,  the  scorification  losses  may  be  considerable,  owing  to  the 
retention  in  the  slag,  or  on  the  sides  of  the  scorifier,  of  undecomposed  silver 
minerals. 

The  scorification  assay  is  simple,  inexpensive  and  reasonably 
rapid.  For  the  determination  of  silver  in  sulphide  ores  having 
an  acid  gangue,  it  is  generally  satisfactory  and  widely  used.  It 
is  particularly  suited  for  the  determination  of  silver  in  ores  con- 
taining considerable  amounts  of  the  sulphides,  arsenides  or  an- 
timonides  of  the  difficultly  oxidizable  base  metals,  particularly 


134  A   TEXTBOOK  OF  FIRE  ASSAYING 

copper,  nickel  and  cobalt.  It  is  used  in  many  localities  for  sil- 
ver in  all  sulphide  ores,  as  well  as  for  gold  and  silver  in  copper 
bullion,  impure  lead  bullion,  copper  and  nickel  mattes  and  speiss. 

It  is  not  to  be  recommended  for  pure  ores,  low  in  silver,  be- 
cause of  the  difficulty  of  handling  and  weighing  the  small  beads 
obtained.  Rich  ores  have  to  be  weighed  more  carefully  for  scor- 
ification  assays,  than  for  crucible  assays  where  usually  two  and  a 
half  to  five  times  as  much  ore  is  used,  in  order  to  obtain  the 
same  precision.  The  ordinary  charge  using  0.10  assay-ton  of 
ore,  does  not  give  a  close  enough  approximation  on  a  gold  ore 
for  commercial  purposes.  Therefore,  in  the  case  of  ores,  this 
method  is  restricted  to  those  containing  only  silver.  Ores  con- 
taining both  silver  and  gold  will  ordinarily  be  assayed  by  the 
crucible  method  so  as  to  obtain  gold  results  of  the  necessary 
precision. 

There  is  no  good  reason  for  scorifying  ores  or  products  which 
do  not  require  oxidation,  and  scorification  is  entirely  unfitted 
for  those  ores  carrying  higher  oxides  such  as  magnetite,  hema- 
tite, pyrolusite,  etc.  It  is  not  suitable  for  ores  having  any  con- 
siderable amount  of  basic  gangue  as  but  a  small  quantity  of 
acid  reagents  can  be  used.  It  should  not  be  used  on  ores  con- 
taining volatile  constituents  such  as  carbonates  and  minerals  con- 
taining water  of  crystallization  which  tend  to  cause  spitting  and 
consequent  loss  of  alloy,,  Volatile  compounds  of  the  precious 
metals  are  more  likely  to  escape  from  a  scorifier  than  from  a 
crucible  because  of  the  exposed  conditions  of  the  ore  in  the  former. 

Chemical  Reactions  in  Scorification.  REACTIONS  DUE  TO  HEAT 
ALONE.  — Various  chemical  changes  may  be  caused  by  heat  alone, 
so  that  during  the  fusion  period,  even  in  the  absence  of  oxygen, 
the  hydrates  give  up  their  water,  most  of  the  carbonates  give 
up  their  carbon  dioxide  and  are  converted  into  oxides  and  even 
some  of  the  sulphates  are  decomposed. 

Chalcopyrite  breaks  up  as  follows  when  heated  to  200°  C. : 

2  Cu  FeS2  =  Cu2S  +  2  FeS  +  S. 

As  soon  as  the  temperature  rises  above  540°  C.  the  iron  and  cop- 
per sulphides  melt,  forming  matte. 

Pyrite,  when  heated  to  redness,  is  decomposed  about  as 
follows : 

7  FeS2  =  Fe7S8  +  6  S. 


THE  SCORIFICATION  ASSAY  135 

The  exact  composition  of  the  residual  iron  sulphide  depends 
upon  the  temperature  and  the  partial  pressure  of  the  sulphur 
vapor.  For  all  practical  purposes  the  reaction  may  be  written 

FeS2  =  FeS  +  S. 

During  the  fusion  stage  the  lead  melts  and  reacts  with  any 
silver  sulphide  which  may  be  present,  as  follows: 

Ag2S  +  Pb  =  2  Ag  +  PbS. 

The  metallic  silver  is  immediately  dissolved  by  the  excess  of 
molten  lead. 

SLAG  FORMING  REACTIONS. — According  to  the  evidence  of 
freezing-point  diagrams  a  few  simple  combinations  of  silica  and 
the  various  metallic  oxides  form  compounds.  So  we  are  justified 
in  writing  reactions  such  as  the  following: 

2  PbO  +  SiO2  =  Pb2SiO4. 

This  may  be  termed  a  slag-forming  reaction  but,  in  general, 
slags,  as  far  as  we  know,  are  igneous  solutions  of  one  constituent 
oxide  in  another.  In  the  molten  state  they  follow  the  laws  of 
solutions  and  should  be  so  considered. 

Most  chemical  reactions  cause  either  an  evolution  or  an  ab- 
sorption of  a  considerable  quantity  of  heat,  but,  from  what  little 
evidence  we  have,  the  heats  of  formation  of  silicates  and  borates 
from  their  component  oxides  is  very  small  and  it  is  doubtful 
whether  these  combinations  should  be  termed  reactions. 

SIMPLE  OXIDATION.  —  As  soon  as  the  air  is  admitted  to  the 
muffle,  the  lead  begins  to  oxidize  to  PbO,  and  this  oxidation  con- 
tinues through  the  whole  scorification  period. 

ROASTING  REACTIONS.  —  The  sulphides  in  the  ore  are  roasted 
as  indicated  by  the  following  reactions: 

FeS  +  3O  =  FeO  +  SO2, 

PbS  +  3O  =  PbO  +  SO2, 

2PbS  +  7O  =  PbO  +  PbSO4  +  SO2, 

ZnS  +  30  =  ZnO  +  S02, 

Sb2S3  +  90  =  Sb2O3  +  3SO2. 

Part  of  the  Sb20s  is  volatilized,  and  part  of  it  is  oxidized  to 
Sb2O5  and  combines  with  litharge,  forming  lead  antimonates, 
£PbO.2/Sb2O5.  Arsenic  behaves  much  like  antimony. 

The  roasting  reactions  shown  above  are  exothermic  and,  owing 


136  A   TEXTBOOK  OF  FIRE  ASSAYING 

to  the  escape  of  the  sulphur  dioxide,  proceed  rapidly  in  a  right- 
handed  direction. 

REACTIONS  BETWEEN  SULPHIDES  AND  OXIDES. — After  enough 
PbO  has  been  formed  to  slag  the  siliceous  gangue,  the  litharge 
which  is  formed  reacts  on  the  partially  decomposed  sulphides, 
aiding  in  the  elimination  of  sulphur,  thus: 

PbS  +  2PbO  =  3Pb  +  S02j 
ZnS  +  3PbO  =  3Pb  +  ZnO  +  SO2, 
Ag2S  +  2PbO  =  2Pb.Ag  +  SO2, 
AsaSa  +  9PbO  =  As2O3  +  3S02  +  9Pb. 

Lead  sulphate  also  reacts  with  lead  sulphide  as  indicated  by  the 
following  reactions: 

PbS  +  PbSO4  =  2Pb  +  2S02, 

PbS  +  2PbS04  =  Pb  +  2PbO  +  3S02, 

PbS  +  3PbSO4  =  4PbO  +  4SO2. 

The  double  reactions  shown  above  are  endothermic,  and  hence 
are  probably  relatively  unimportant  in  scorification. 

If  Cu2S  were  present  in  the  ore,  part  of  it  would  be  oxidized  to 
CuO,  and  then  the  cuprous  sulphide  and  the  cupric  oxide  would 
tend  to  react  as  follows : 

Cu2S  +  2CuO  =  4Cu  +  SO2. 

A  similar  reaction  between  the  litharge  and  the  cuprous  sul- 
phide would  probably  take  place  as  follows : 

Cu2S  +  2PbO  =  2CuPb  +  SO2. 

A  prolonged  scorification  is  required  to  remove  the  copper  thus 
reduced  and  alloyed  with  the  lead.  The  last  two  reactions  are 
more  pronounced  at  high  temperatures,  so  that  for  the  elimination 
of  copper  in  the  scorification  assay  it  is  evident  that  a  low  muffle 
temperature  should  be  maintained. 

Indications  of  Metals  Present.  —  The  color  of  the  thin  coating 
of  slag  on  the  scorifier  is  an  indication  of  the  amount  and  kind  of 
metal  originally  present  in  the  ore,  and  taken  in  connection  with 
the  mineralogical  examination  of  the  ore  it  gives  a  very,  good 
approximation  as  to  its  composition. 

COPPER  gives  a  light  or  dark  green,  depending  on  the  amount 
present.  If  there  is  much  iron  in  the  ore  this  color  may  be  wholly 
or  in  part  obscured  by  the  black  of  the  iron  oxide.  Practically 


THE  SCORIFICATION  ASSAY  137 

all  of  the  iron  is  removed  in  the  first  scorification,  so  that  in  assay- 
ing a  copper  matte  the  first  scorifier  may  appear  black  while  the 
second  one  will  be  green.  The  green  color  is  said  to  be  due  to  a 
mixture  of  blue  cupric  silicate  and  yellow  lead  silicate. 

IRON.  —  A  large  amount  of  iron  makes  the  scorifier  black,  from 
which  the  color  ranges  from  a  deep  red  through  various  shades  of 
brown  to  a  yellow  brown. 

LEAD,  in  the  absence  of  other  metals,  makes  the  scorifier  lemon- 
yellow  to  a  very  pale  yellow. 

COBALT  gives  a  beautiful  blue  if  other  metals  do  not  interfere. 

NICKEL  colors  the  scorifier  brown  to  black  depending  on  the 
amount  present.  When  much  nickel  is  present  the  cupel  becomes 
covered  with  a  thick  film  of  green  nickel  oxide. 

MANGANESE  colors  the  scorifier  brownish-black  to  a  beautiful 
wine-color. 

ARSENIC  and  ANTIMONY,  if  present  in  large  amounts,  will  leave 
crusts  on  the  inner  surface  of  the  scorifier  even  if  much  borax-glass 
is  used.  In  the  absence  of  other  metals  the  scoria  will  be  yellow 
in  color. 

If  a  scorifier  is  colored  dark  green,  indicating  much  copper,  dark 
blue,  indicating  much  cobalt,  or  black  with  infusible  scoria,  in- 
dicating nickel,  the  button  should  be  scorified  again  with  more 
lead. 

Rescorifying  Buttons.  —  When  it  is  necessary  to  rescorify  but- 
tons to  remove  copper  or  other  impurities,  or  when  bullion  is 
assayed  by  scorification,  a  good  plan  is  to  place  the  scorifiers  con- 
taining the  right  amount  of  test  lead  in  a  hot  muffle.  When  the 
molten  lead  has  ceased  spitting,  the  button,  or  bullion,  is  dropped 
in.  This  precaution  is  suggested  to  prevent  loss  of  bullion  by 
spitting  which  occurs  quite  often  in  rescorifying,  probably  -be- 
cause of  moisture  in  the  scorifier.  Another  method  is  to  place 
the  scorifier  in  the  muffle  and  heat  for  ten  or  fifteen  minutes  and 
then  drop  in  the  buttons  and  the  proper  amount  of  lead. 

Buttons  weighing  over  35  grams  should  be  scorified  to  15  or 
20  grams-  before  being  cupeled.  If  this  is  carefully  done,  the  loss 
of  silver  should  be  less  by  the  combined  method  than  by  direct 
cupellation. 

Spitting  of  Scorifiers.  —  Occasionally  small  particles  of  lead 
are  seen  being  projected  out  of  the  scorifier.  This  is  due  to  de- 
crepitation of  the  ore  or  to  the  action  of  some  gas  given  off  by  the 


138  A   TEXTBOOK  OF  FIRE  ASSAYING 

ore  or  scorifier  itself.  If  the  particles  of  lead  do  not  all  fall  back 
into  the  scorifier  a  loss  of  precious  metal  will  result.  The  direct 
cause  may  be  found  among  the  following  and  a  proper  remedy 
applied : 

1.  Dampness  of  scorifier. 

2.  Presence  of  carbonates  in  clay  from  which  scorifier  was 
made. 

3.  Imperfect  mixing  of  charge,  resulting  in  ore  being  left  on 
the  bottom  of  the  scorifier  and  covered  with  lead. 

4.  Too  high  a  temperature  at  the  start,  resulting  in  too  rapid 
oxidation  of  sulphides,  evolution  of  CC>2  or  violent  decrepitation. 

5.  Admittance  of  air  into  the  muffie  too  soon,  resulting  in  too 
rapid  oxidation.     (Especially  to  be  avoided  in  the  case  of  ores  or 
products  carrying  zinc.) 

6.  Character  of  the  ore  itself.     (Ores  containing  carbonates 
etc.,  are  not  suited  for  scorifi cation.) 

Assaying  Granulated  Lead.  —  Almost  all  assay  reagents  con- 
tain traces  of  gold  and  silver,  but  the  lead  and  litharge  are  es- 
pecially likely  to  contain  these  metals  in  appreciable  amounts. 
Each  new  lot  of  granulated  lead  which  is  obtained  should  be 
sampled  and  assayed  before  it  is  used,  and  in  case  any  silver  or 
gold  is  found  a  strict  account  must  be  kept  of  the  lead  used  in 
each  assay  and  a  correction  for  its  precious  metal  contents 
made. 

PROCEDURE.  —  Scorify  2  or  3  portions  of  120  grams  each  in 
3J  or  4  inch  scorifiers.  If  necessary  rescorify  until  the  buttons 
are  reduced  to  15  or  20  grams.  Cupel,  weigh  and  part.  This 
correction  must  be  made  even  if  extremely  small,  as  any  error 
thus  introduced  would  be  multiplied  by  10  in  reporting  the  re- 
sults in  ounces  per  ton. 

Scorification  Assay  for  Gold.  —  The  silver  in  an  ore  can  be 
determined  with  a  sufficient  degree  of  accuracy  by  taking  0.1 
A.  T.  for  each  assay,  since  we  may  thus  determine  the  contents 
of  the  ore  to  0.1  of  an  ounce,  or  its  value  to  5  or  10  cents  a  ton. 
When,  however,  we  determine  gold  to  0.1  ounce  per  ton  by  this 
same  method,  we  have  determined  its  value  to  only  $2.00  per  ton, 
which  is  not  sufficiently  accurate  for  any  but  very  high-grade 
ores.  For  this  reason  the  scorification  assay  is  not  usually  chosen 
for  gold  ores  unless  they  contain  impurities  which  interfere  seri- 
ously with  the  crucible  assay. 


THE  SCOR1FICATION  ASSAY  139 

SCORIFICATION   ASSAY   OF   COPPER   MATTE. 

Procedure.  —  Take  three  portions  of  0.1  A.  T.  of  matte,  mix 
with  45  grams  of  granulated  lead  and  1  gram  powdered  silica  in  a 
3-inch  Bartlett  scorifier,  and  cover  with  60  grams  more  of  lead. 
Put  half  a  gram  of  borax-glass  and  1  gram  of  silica  on  top.  Scor- 
ify hot  at  first  and  then  at  a  low  temperature  to  facilitate  slagging 
the  copper. 

When  the  lead  eye  covers,  pour  as  usual  and  separate  the  lead 
from  the  slag.  Weigh  each  button  and  add  sufficient  granulated 
lead  to  bring  the  total  weight  to  60  grams  and  drop  into  three 
new  scorifiers  which  have  been  heated  in  the  muffle.  Add  about 
1  gram  of  silica  and  scorify  at  a  low  temperature. 

If  necessary,  repeat  this  second  scorification  until  the  cool 
scorifiers  are  light-green.  Cupel  as  usual.  The  color  of  the  cu- 
pel should  be  greenish  and  not  black.  The  latter  color  indicates 
insufficient  scorification. 

Weigh  the  combined  silver  and  gold  and  part,  weighing  the  gold. 

Notes:  1.  For  matte  containing  not  more  than  30  per  cent  of  coppe 
two  scorifications  are  sufficient. 

2.  This  method  gives  rather  high  slag  and  cupel  losses  and  for  exact 
work  the  slags  and  cupels  are  reassayed  and  a  correction  made  for  their  silver 
and  gold  contents. 

3.  The  final  silver  beads  will  often   contain  from   2   to  4  per   cent  of 
copper. 

4.  When  accurate  results  in  gold  are  desired,  as  many  as  10  portions  of 
0.1  A.  T.  each  of  matte  are  scorified  and  the  buttons  combined  for  .parting  and 
weighing. 

Losses  in  Scorification.  —  Losses  in  scorification  may  be  due  to 
"  spitting,"  volatilization,  oxidation  and  slagging  as  well  as  to 
shots  of  alloy  lost  in  pouring.  Some  loss  due  to  oxidation  and 
slagging  is  unavoidable,  but  it  should  be  low.  If  there  is  any 
decided  loss  by  volatilization  it  shows  that  the  process  is  un- 
suited  to  the  ore. 

The  tendency  of  scorification  assays  to  "spit"  is  one  of  the 
most  serious  objections  to  the  process.  Ores  which  decrepitate 
or  contain  volatile  constituents  such  as  CO2,  H2O,  etc.,  (CaCO3, 
CaSO4.2H2O)  are  unsuited  to  the  process  and  should  be  assayed 
by  crucible  methods.  Very  often  a  preliminary  glazing  of  the 
scorifier  with  a  mixture  of  sodium  carbonate  and  borax-glass  will 
prevent  spitting.  The  scorifiers  should  always  be  kept  in  a 
warm,  dry  place. 


140 


A   TEXTBOOK  OF  FIRE  ASSAYING 


Losses  of  alloy,  due  to  failure  of  all  the  lead  to  collect  in  one 
piece,  may  be  caused  by  careless  pouring,  in  which  case  some  of 
the  lead  may  splash  on  the  side  of  the  mold  and  solidify  there, 
or  by  a  poor  slag,  or  a  cold  pour,  resulting  in  shots  of  alloy 
being  left  in  the  scorifier  or  scattered  through  the  slag  in  the  mold. 

As  scorification  is  an  oxidizing  process  it  is  only  reasonable  to 
expect  some  loss  due  to  oxidation  of  the  precious  metals,  and 
this  will  naturally  be  greater  the  longer  the  scorification  is  con- 
tinued and  the  more  intense  the  oxidizing  action.  Silver  is  more 
easily  oxidized  than  gold,  therefore  we  should  expect  a  much 
greater  loss  of  silver  than  of  gold.  To  keep  this  loss  at  a  minimum 
let  the  liquefaction  period  be  thorough.  The  molten  lead  tends  to 
reduce  and  collect  some  of  the  silver  previously  slagged.  Some  as- 
sayers  recommend  sprinkling  a  small  amount  of  charcoal  over  the 
slag  in  the  scorifier  just  before  closing  the  door  of  the  muffle  for  the 
liquefaction  period,  with  the  idea  of  reducing  some  lead  from  the 
slag  and  thus  collecting  most  of  the  oxidized  silver  by  the  rain  of  lead 
shot  thus  induced.  English  authorities  almost  invariably  recom- 
mend this  practice  which  they  term  "  cleaning  the  slag." 

Keller*  gives  average  figures  for  corrected  assays  on  anode 
mud  known  to  contain  3750  ounces  of  silver  per  ton.  Assays 
were  by  scorification  and  in  one  series  scorifications  and  cupel- 
lations  were  run  hot  while  in  the  other  they  were  run  cool. 
The  results,  each  representing  an  average  of  twenty  individual 
assays,  are  shown  in  the  following  table: 

TABLE  XVII. 
ASSAYS  OF  COPPER  ANODE  RESIDUES. 


Origin 

Hot  scorification 
and  cupellation 

Cool  scorification 
and  cupellation 

Silver  oz. 
per  ton 

Gold  oz. 
per  ton 

Silver  oz. 
per  ton 

Gold  oz. 
per  ton 

In  beads  

3613.12 
55.38 
21.64 

28.030 
0.010 
0.045 
0.075 

3688.85 
56.44 
20.93 

27.815 

0.020 
0.025 
0.225 

In  slag.     

In  cupels  

In  decantation 

Total   . 

3690.14 

28.160 

3766.22 

28.085 

*  Trans.  A.I.M.E.,  60,  p.  706. 


THE  SCORIFICATION  ASSAY  141 

The  greatest  difference  between  hot  and  cool  assays  is  shown 
in  the  uncorrected  assay  results.  The  other  figures  agree  sur- 
prisingly well.  The  loss  of  silver  shown  by  these  figures  is  very 
high,  due  to  the  repeated  scorifications  necessary  and  the  effect 
of  copper  in  increasing  the  loss.  The  loss  of  gold  is  extremely 
small  and  serves  to  illustrate  the  protective  action  of  silver  on 
gold.  » 

The  difference  in  temperature  of  hot  and  cool  cupellations 
could  not  have  been  great,  or  else  the  cool  scorification  gave 
purer  buttons  for  cupellation,  as  the  cupel  losses  differ  very 
little. 

Because  of  the  low  results  of  the  corrected  assays,  in  the  case 
of  hot  scorifications  and  cupellations,  compared  with  the  known 
silver  content  of  3750  ounces  per  ton,  Keller  concludes  that  there 
must  have  been  a  decided  loss  of  silver  by  volatilization.  This 
is  a  good  argument  for  cool  scorification  in  copper  work  as  well 
as  for  cool  cupellation. 

The  gold  lost  in  decantation,  in  the  case  of  beads  resulting 
from  cool  work,  is  three  times  that  for  beads  resulting  from  hot 
work.  This  difference  he  claims  to  be  due  to  increased  disintegra- 
tion of  the  gold,  because  of  the  presence  of  added  impurities  re- 
tained in  the  beads  resulting  from  cool  cupellation. 

Use  of  Large  Ore  Charges  in  Scorification.  —  While  the  usual 
charge  for  a  scorification  assay  is  0.1  assay-ton,  Simonds*  claims 
to  be  able  to  obtain  good  results  on  practically  all  classes  of  sul- 
phide ores  using  0.5  assay-ton  of  ore,  75  grams  of  lead  and  2.5 
grams  of  borax-glass  in  a  3-inch  shallow  scorifier.  This  should 
certainly  cause  no  difficulty  with  mixtures  consisting  only  of 
galena  and  quartz. 

Scorification  Charges  for  Different  Materials.  —  The  follow- 
ing charges  have  been  found  generally  satisfactory: 

*  California  Mines  and  Minerals,  p.  226  (1899). 


142 


A   TEXTBOOK  OF  FIRE  ASSAYING 


TABLE  XVIII. 

SCORIFICATION   CHARGES. 


Char 

?e 

Material 

• 

Ore 

Assay 
Tons 

Granu- 
lated 
lead 
Grams 

Borax- 
glass 
Grams 

Silica 
Grams 

Scorifier 
Inches 

Heat 

high  at 
first 
then 

Galena.  .    . 

0.1 

35 

i-i 

2J 

Low 

Half  galena,   half 
silica  

0.1 

35 

H 

2x 

Low 

Low  grade  galena  . 
Pyrite  

0.2 
0.1 

45 
50 

H 

2-3 

- 

2l 

2f 

Low 
Medium 

Half    pyrite,    half 
silica  
Stibnite  
Sphalerite  
Arsenical  ore  .... 
Cobalt  ore 

0.1 
0.1 
0.1 
0.1 
0.1 

45 
50-60 
60 
45-60 
60 

1-2 
1-2 
3-5 
1-2 
3 

1-2 

21 

2f-3 
3 
2f-3 

3 

Medium 
High 
High 
High 
High 

Nickel  ore  . 

0.05-0.1 

60 

3 

_ 

3 

High 

Chalcopyrite  

0.1 

60 

1-2 

1 

3 

Low 

Tin  ore  

0.1 

60-70 

2-3 

1 

3-3£ 

High 

Lead  matte  
Copper  matte  

0.1 
0.1 

50 
60 

i 

1 

2| 

3 

Low 
Low 

CHAPTER  VIII. 
THE  CRUCIBLE  ASSAY. 

Theory  of  the  Crucible  Assay.  —  The  majority  of  ores  are, 
by  themselves  infusible,  or  nearly  so,  but  if  pulverized  and 
mixed  in  proper  proportion  with  suitable  reagents,  the  mix- 
ture will  fuse  at  an  easily  attained  temperature.  The  finer 
the  ore _isjcrushed,  the  better  and  more  uniform  are  the  re- 
sults obtained.  We  assume  in  considering  a  crucible  assay  that 
there  is  such  a  thorough  mixture  of  ore  ^and  fluxes  that  each 
particle  of  ore  is  in  contact  with  one  or  more  particles  of  litharge 
and  reducing  agent.  As  the  temperature  of  the  mass  is  gradually 
raised,  part  of  the  litharge  is  reduced  to  lead  (commencing  at 
500°  to  550°  C.)  by  the  carbon  of  the  charge,  and  these  reduced 
shots  of  lead,  alloy  and  take  up  the  gold  and  silver  from  the  sur- 
rounding particles  of  ore,  so  far  at  least  as  the  precious  metals 
are  free  to  alloy. 

At  about  this  same  temperature,  560°  C.,  the  borax  of  the 
charge  begins  to  melt  and  to  form  fusible  compounds  with  some 
of  the  bases  of  the  flux  and  ore  charge.  In  the  absence  of  borax 
or  other  fusible  constituents,  lead  oxide  and  silica  commence 
to  combine  at  about  700°  C.,  and  from  this  point  the  slag 
begins  to  form  rapidly.  The  conditions  should  be  such  that 
the  slag  remains  viscous  until  the  ore  particles  are  thoroughly 
decomposed  and  every  particle  of  gold  and  silver  has  been  taken 
up  by  the  adjacent  suspended  globules  of  lead.  After  this  point 
has  been  passed,  the  temperature  may  be  raised  until  the  slag 
is  thoroughly  fluid,  when  the  lead  particles  combine  and,  falling 
through  the  slag,  form  a  button  in  the  bottom  of  the  crucible 
in  which  are  concentrated  practically  all  of  the  precious  metals 
originally  present  in  the  ore. 

To  make  an  intelligent  crucible  assay  it  is  necessary  to  know 
the  mineral  character  of  the  ore,  for  a  siliceous  ore  requires  a 
different  treatment  from  one  which  is  mostly  limestone  and  a 
sulphide  requires  to  be  treated  differently  from  an  oxide.  For 
the  purpose  of  the  assayer,  ores  should  be  considered  from  two 

143 


144  A   TEXTBOOK  OF  FIRE  ASSAYING 

standpoints,  first  according  to  the  character  and  quantity  of 
their  slag-forming  constituents,  and  second  according  as  they  are 
oxidizing,  neutral  or  reducing  in  the  crucible  fusion  with  lead  and 
lead  oxide. 

Ores  Classified  According  to  Slag-forming  Constituents.  —  The 
principal  slag-forming  constituents  of  ores  and  gangue  minerals, 
arranged  approximately  in  the  order  of  their  occurrence  in  the 
earth's  crust  are  as  follows: 


Silica  Si02    1     A   . , 

AI  AI  r\   I    Acids 

Alumina  A1203 

Ferrous  oxide  FeO 

Manganous  oxide  MnO 

Calcium  oxide  CaO 

Magnesium  oxide  MgO 

Sodium  oxide  Na2O 

Potassium  oxide  K2O 

Zinc  oxide  ZnO 

Lead  oxide  PbO 

Cuprous  oxide  Cu2O 


These  oxides,  with  the  exception  of  those  of  sodium,  potassium 
and  lead,  are  infusible  at  the  temperature  of  the  assay-furnace. 
To  get  them  into  the  molten  condition  we  add  fluxes.  All  of  the 
common  assay  fluxes  with  the  exception  of  silica  are  readily 
fusible  by  themselves.  In  general  it  may  be  said  that  to  flux  the 
acid,  silica,  it  is  necessary  to  add  bases  and  to  flux  any  of  the 
basic  oxides,  acids  must  be  added.  To  flux  alumina  it  is  best 
to  add  both  acids  and  bases. 

Ores  Classified  According  to  Oxidizing  or  Reducing  Charac- 
ter. —  According  to  their  oxidizing  or  reducing  character  in  the 
crucible  assay,  ores  may  divided  into  three  classes  as  follows: 

CLASS  1.  NEUTRAL  ORES.  —  Siliceous,  oxide  and  carbonate 
ores  or  ores  containing  no  sulphides,  arsenides,  antimonides, 
tellurides,  etc.,  i.e.,  ores  having  no  reducing  or  oxidizing  power. 

CLASS  2.  ORES  HAVING  A  REDUCING  POWER. —  Ores  containing 
sulphides,  arsenides,  antimonides,  tellurides,  carbonaceous  matter, 
etc.,  or  ores  which  decompose  litharge  with  a  reduction  of  lead 
in  the  crucible  fusion. 


THE  CRUCIBLE  ASSAY  145 

CLASS  3  ORES  HAVING  AN  OXIDIZING  POWER.  —  Ores  con- 
taining ferric  oxide,  manganese  dioxide,  etc.,  or  ores  which  when 
fused  with  fluxes  oxidize  lead  or  reducing  agents.  Ores  with 
any  considerable  oxidizing  power  are  comparatively  rare. 

Determining  the  Character  of  a  Sample.  —  The  mineral  char- 
acter of  an  ore  can  be  most  readily  determined  when  the  ore  is  in 
the  coarse  condition.  However,  as  a  large  proportion  of  the  sam- 
ples received  by  the  assayer  are  already  pulverized,  it  becomes 


FIG.  43.  —  Fan  of  galena  and  quartz  on  vanning-shovel. 

necessary  for  him  to  be  able  to  form  a  close  estimate  of  their 
composition  in  this  condition.  This  may  be  best  accomplished 
by  washing  a  small  sample  on  a  vanning-plaque  or  shovel. 

Place  one  or  two  grams  of  the  ore  on  the  vanning-shovel,  cover 
it  with  water  and  allow  it  to  stand  until  the  ore  is  thoroughly 
wet,  then  shake  violently  in  a  horizontal  plane  until  the  fine 
slime  is  in  suspension  and  all  lumps  are  broken  up.  Allow 
to  settle  a  moment,  decant  some  of  the  water  if  necessary  and  then 


146  A   TEXTBOOK  OF  FIRE  ASSAYING 

separate  the  ore  according  to  the  specific  gravity  of  its  different 
minerals  by  a  combined  washing  and  shaking.  The  water  should 
be  made  to  flow  over  the  ore  in  one  direction  only  and  the  velocity 
of  the  shaking  motion  should  be  accelerated  in  a  direction  opposite 
to  the  flow  of  the  water.  The  shaking  tends  to  stratify  the  ore, 
heaviest  next  the  pan,  lightest  on  top,  while  the  water  tends  to  wash 
everything  downward,  the  material  on  top  being  most  affected  be- 
cause of  its  position,  and  also  because  of  its  lesser  specific  gravity. 
Finally,  if  there  are  a  number  of  minerals  present,  they  should 
appear  spread  out  in  fan  shape  in  the  order  of  their  specific  grav- 
ity, for  instance,  galena,  pyrite,  sphalerite  and  quartz. 

Figure  43  shows  a]fan  of  galena  and  quartz  on  a  vanning-shovel. 
If  account  is  taken  of  the  specific  gravity  of  the  different  min- 
erals, an  experienced  operator  can  make  a  reasonably  good  esti- 
mate of  the  percentage  of  each  of  the  common  minerals  present 
in  an  ore. 

Crucible  Slags.  —  The  slags  obtained  in  the  crucible  assay 
may  be  regarded  as  silicates  and  borates  of  the  metallic  oxides 
dissolved  in  one  another  and  in  litharge.  They  also  often  con- 
tain dissolved  carbon  dioxide.  The  acid  constituents  of  rocks, 
other  than  silica,  so  seldom  play  an  important  part  in  the  forma- 
tion of  slags  that  they  may  be  omitted  at  least  from  a  preliminary 
discussion  of  the  subject. 

Assay  slags  high  in  litharge  and  low  in  silica,  borax  and  other 
acids  are  sometimes  called  oxide  slags.  Very  little  is  known 
about  the  constitution  of  these  slags. 

Very  acid  slags  are  sometimes  emulsions  and  not  true  solutions. 
That  is,  they  may  contain  suspended  solid  or  molten  globules 
of  other  minerals. 

A  slag  suitable  for  assay  purposes  should  have  the  following 
properties : 

1.  It  should  have  a  comparatively  low  formation  temperature 
readily  attainable  in  assay-furnaces. 

2.  It  should  be  pasty  at  and  near  its  formation  temperature, 
to  hold  up  the  particles  of  reduced  lead  until  the  precious  metals 
are  liberated  from  their  mechanical  or  chemical  bonds  and  are 
free  to  alloy  with  the  lead. 

3.  It  should  be  thin  and  fluid  when  heated  somewhat  above 
its  melting-point,  so  that  shots  of  lead  may  settle  through  it 
readily. 


THE  CRUCIBLE  ASSAY  147 

4.  It  should  have  a  low  capacity  for  gold  and  silver,  and  should 
allow  a  complete  decomposition  of  the  ore  by  the  fluxes. 

5.  It  should  not  attack  the  material  of  the  crucible  too  vio- 
lently. 

6.  Its  specific  gravity  should  be  low,  to  allow  a  good  separa- 
tion of  lead  and  slag. 

7.  When  cold,  it  should  separate  readily  from  the  lead  and 
be  homogeneous,  thus  indicating  complete  decomposition  of  the 
ore. 

8.  It  should  contain  all  the  impurities  of  the  ore  and  should 
be  free  from  the  higher  oxides  of  the  metals. 

9.  Except  in  the  case  of  the  iron-nail  assay  it  should  be  free 
from  sulphides. 

Color  of  Crucible  Slags.  —  As  is  also  the  case  in  scorification, 
the  color  of  the  slags  resulting  from  crucible  assays  is  often  in- 
dicative of  the  metals  present.  Due  to  the  larger  proportion 
of  silica  and  borax  and  the  smaller  amount  of  litharge  in  crucible 
slags  the  significance  of  the  coloring  is  not  always  the  same. 
Thus  in  crucible  slags  various  shades  of  green  may  usually  be 
ascribed  to  the  presence  of  ferrous  silicates  and  not  of  copper 
as  is  the  case  in  scorification  slags,  while  in  the  absence  of  iron, 
copper  gives  the  crucible  slag  a  brick-red  color,  due  to  the  pres- 
ence of  cuprous  silicate  or  borate. 

Calcium,  magnesium,  aluminum  and  zinc  give  white  or  gray- 
ish-white slags,  usually  more  or  less  opaque.  The  acid  silicates 
of  pure  soda  and  lead  are  clear,  colorless  glasses.  Cobalt  gives 
the  well-known  cobalt  blue.  Iron  and  manganese  in  large  quanti- 
ties make  the  slag  black.  A  small  amount  of  manganese  may, 
in  the  absence  of  interfering  elements,  yield  a  purple  to  a  light 
pink;  and  as  is  well  known  by  all  glass-makers,  a  small  amount 
serves  to  neutralize  the  color  effect  of  iron. 

Classification  of  Silicates.  —  Silicates  are  classified  accord- 
ing to  the  proportion  of  the  oxygen  in  the  acid  to  oxygen  in  the 
base.  Thus,  a  mono-silicate  has  the  same  amount  of  oxygen 
in  the  acid  as  in  the  base.  A  bi-silicate  has  twice  as  much  oxy- 
gen in  the  acid  as  in  the  base  and  so  on. 

The  silicates  which  have  been  found  to  behave  satisfactorily 
as  assay  slags  lie  within  the  following  limits: 


148 


A   TEXTBOOK  OF  FIRE  ASSAYING 


TABLE  XIX. 

CLASSIFICATION  OF  SILICATES. 


Oxygen  ratio 

Formula. 

Acid  to  base 

R  =  bivalent  base 

Sub-silicate 

£tol 

4RO.SiO2 

Mono-  or  singulo-silicate 
Sesqui-silicate 
Bi-silicate 

1  to  1 
U  to  1 
2  to  1 

2RO.SiO2 
4RO.3SiO2 
RO.SiO2 

Tri-silicate 

3  to  1 

2RO.3SiO2 

The  formation  temperature  and  melting-point  of  the  different 
silicates  depend  not  only  on  the  relation  of  the  silica  to  base, 
but  also  on  the  nature  of  the  bases  present.  Thus  we  may  say 
that  within  the  above  range  the  silicates  of  lead  and  the  alkalies 
are  all  readily  fusible,  the  iron  and  manganese  silicates  are  diffi- 
cultly fusible  and  the  silicates  of  calcium,  magnesium  and  alu- 
minum are  infusible  at  the  temperature  of  the  assay-furnace. 
Note  that  so  far  we  are  referring  to  the  individual  silicates  of 
the  different  bases  and  not  to  mixtures  of  the  same. 

Of  these  slags  the  bi-  and  the  tri-silicates  have  but  little  effect 
on  the  ordinary  assay  crucible  while  the  sub-silicates  attack  it 
strongly  to  satisfy  their  affinity  for  silica. 

The  student  should  distinguish  between  the  formation  tempera- 
ture of  a  slag  and  the  melting-point  of  the  same  slag  when  al- 
ready formed.  It  has  been  shown  by  Day,*  that  when  the 
constituents  of  a  slag  are  finely  crushed  and  intimately  mixed 
as  in  an  assay  fusion,  the  formation  temperature  of  the  slag  is 
decidedly  lower  than  the  melting  temperature.  That  is  to  say, 
the  slag  forms  without  melting  and  actually  passes  through  a 
pasty  stage  before  coming  to  perfect  fusion. 

Action  of  Borax  in  Slags.  —  Borax  (Na2B4O7  +  10H2O)  melts 
at  about  560°  C.  and  gives  up  its  water  of  crystallization  forming 
borax-glass.  Borax-glass  when  molten  is  decidedly  viscous,  and 
on  account  of  its  excess  of  boric  oxide  acts  as  an  acid  flux. 

Although  primarily  an  acid  flux,  borax  exerts  considerable 
solvent  power  upon  the  silicates  as  well.  It  lowers  the  tempera- 
ture of  slag  formation  and  in  the  case  of  non-sulphide  ores  helps 
to  make  the  slag  viscous  during  the  reduction  period.  In  the 

*  Jour.  Am.  Chem.  Soc.,  28,  p.  1039  (Sept.  1906). 


THE  CRUCIBLE  ASSAY  149 

case  of  basic  ores  particularly,  it  reduces  the  temperature  of 
final  complete  fluidity.  According  to  Steel*  it  seems  to  protect 
the  crucibles  from  the  solvent  action  of  litharge,  probably  by 
coating  them  with  a  viscous  aluminum  boro-silicate. 

To  determine  what  relation  it  bears  to  silica  as  regards  its  acid 
fluxing  quality  we  may  consider  the  matter  first  from  a  theoret- 
ical standpoint,  and  then  from  the  results  of  experiments. 

Considering  the  borates  according  to  their  metallurgical  classi- 
fication, i.e.,  according  to  the  amount  of  oxygen  in  the  acid  to 
that  in  the  base,  we  may  compute  the  weight  of  borax-glass 
necessary  to  form  a  mono-borate  with  a  unit  weight  of  sodium 
carbonate  and  compare  this  with  the  amount  of  silica  required 
to  form  a  mono-silicate  with  the  same  amount  of  base.  From 
the  rational  formula  for  borax-glass  (Na2O.2B2O3)  we  see  that  to 
form  the  mono-borate  (6Na2O.2B2O3),  borax-glass  requires  five 
additional  molecules  of  Na2O.  The  equation  may  be  written  as 
follows : 

5Na2C03  +  Na2O.2B2O3  =  6Na2O.2B2O3  +  5CO2. 

Whence  we  may  write  the  following  proportion  to  find  the  amount 
of  borax-glass  necessary  to  form  a  mono-borate  with  100  grams 
of  soda: 

5Na2CO3  :  Na2O.2B2O3  =  530  :  202  =  100  :  x, 

solving,  x  is  found  to  equal  38.1.  In  the  same  way  we  may  find 
the  amount  of  silica  necessary  to  form  a  mono-silicate  with  100 
grams  of  sodium  carbonate, 

2Na2CO3  :  SiO2  =  212  :  60  =  100  :  y, 

solving,  y  is  found  to  equal  28.3.  Whence,  from  the  theoretical 
standpoint  we  may  say  that  in  the  case  of  a  mono-silicate  slag, 
38.1  grams  of  borax-glass  is  equivalent  to  28.3  grams  of  silica, 
or  when  borax-glass  is  used  to  replace  silica  in  a  mono-silicate 
slag  one  gram  has  the  same  effect  as  0.743  grams  of  silica. 

For  a  bi-silicate  slag  the  relation  is  different  owing  to  the  mole- 
cule of  Na2O  already  in  the  borax-glass.  The  amount  of  borax- 
glass  required  to  form  a  bi-borate  with  100  grams  of  sodium  car- 
bonate is  95.3  and  the  silica  for  a  bi-silicate  is  56.6.  Thus,  in 
the  case  of  a  bi-silicate  slag,  1  gram  of  borax-glass  is  equivalent 
to  0.584  grams  of  silica. 

*  Eng.  and  Min.  Jour.,  87,  p.  1243. 


150  A   TEXTBOOK  OF  FIRE  ASSAYING 

Experiments*  on  the  size  of  lead  buttons  obtained  in  reducing 
power  fusions,  with  varying  amounts  of  silica  in  some  instances, 
and  borax-glass  in  others,  give  results  approaching  the  theoretical 
values  obtained  above.  They  show  that  10  grams  of  borax-glass 
has  the  same  effect  in  preventing  the  reduction  of  lead  from  lith- 
arge as  between  6  and  7  grams  of  silica. 

Rose,f  in  a  discussion  of  the  refining  of  gold  bullion  with  oxy- 
gen gas,  made  a  number  of  experiments  to  determine  the  best 
proportions  of  borax,  silica  and  metallic  oxides.  Borax  alone 
was  found  to  be  unsatisfactory  on  account  of  the  rapid  corrosion 
of  the  crucible.  Silica  alone  gave  a  pasty,  very  viscous,  slag. 
The  best  slag  found  corresponded  nearly  to  the  formula  f  (Na2O, 
B203)  +  3RO,  §B203,  3Si02.  This  is  made  up  according  to  the 
following  formula,  9RO  +  2Na2B4O7  +  9SiO2,  where  R  =  Ca,  Mg, 
Pb,  Zn,  Cu,  f  Fe,  f  Ni.  Leaving  out  of  account  the  meta-borate 
of  soda  Na2B204,  it  is  a  boro-silicate  in  which  the  relation  of 
oxygen  in  acids  to  oxygen  in  bases  is  2.66  to  1.  This  slag  melts 
at  a  low  temperature  and  is  very  fluid  at  between  1000°  and  1 100° 
C.  It  has  only  a  slight  corrosive  action  on  clay  crucibles.  The 
flux  contains  3  parts  by  weight  of  borax-glass  to  4  parts  of  silica. 

Charles  E.  MeyerJ  in  fluxing  zinc-box  slime,  made  zinc  into  a 
bi-silicate  with  silica  and  added  Na2B4O7  for  other  bases.  The 
other  bases  were  all  assumed  to  be  Fe2O3  and  borax-glass  was 
added  pound  for  pound,  i.e.,  1  pound  Na2B407  for  1  pound 
Fe203. 

Fluidity  of  Slags.  —  It  is  also  necessary  to  distinguish 
between  the  melting-point  and  the  fluidity  of  slags.  Many 
slags  of  low  melting  and  formation  temperature  are  entirely 
unsuited  for  assay  purposes  on  account  of  their  viscous  nature 
when  melted.  As  a  rule,  the  higher  the  temperature  the  more 
fluid  a  slag  will  become,  but  different  slags  vary  much  in  this 
respect.  All  slags  are  viscous  at  their  freezing-point,  yet  one 
slag  will  be  thinly  fluid  200°  C.  above  its  melting-point  and  another 
will  be  decidedly  viscous  at  this  degree  of  superheat.  The  vis- 
cosity of  silicates  increases  with  the  percentage  of  silica  above  that 
required  for  the  mono-silicate,  and  the  same  may  be  said  for 
borates. 

*  Lodge,  Notes  on  Assaying,  2nd  Ed.  p.. 

t  Inst.  Min.  and  Met.,  14,  p.  396,  (1905). 

j  Jour.  Chem.  Met.  and  Min.  Soc.  of  South  Africa,  5,  p.  168,  (1905). 


THE   CRUCIBLE  ASSAY  151 

Acidic  and  Basic  Slags.  —  Slags  more  acid  than  the  mono-sili- 
cate are  generally  termed  acid,  while  those  approaching  a  sub- 
silicate  are  called  basic.  The  acid  slags  are  all  more  or  less  vis- 
cous when  molten  and  can  be  drawn  out  into  long  threads.  They 
cool  slowly  and  are  usually  glassy  and  brittle  when  cold.  The 
basic  slags  are  usually  extremely  fluid  when  molten;  they  pour 
like  water,  with  no  tendency  to  string  out;  in  fact  they  may  even 
be  lumpy  where  the  bases  are  in  too  great  excess.  They  solidify 
rapidly  and  usually  crystallize  to  some  extent  during  solidifica- 
tion. Basic  slags  are  dull  and  tough  when  cold.  They  are  often 
of  a  dark  color  and  on  account  of  the  large  proportion  of  bases 
they  contain  they  usually  have  a  high  specific  gravity. 

Mixed  Silicates.  —  The  mixture  of  two  or  more  fusible  com- 
pounds usually  fuses  at  a  lower  temperature  than  either  one  taken 
alone,  just  as,  for  example,  a  mixture  of  potassium  and  sodium 
carbonate  fuses  at  a  lower  temperature  than  either  one  of  them 
alone.  For  this  reason  assay ers  always  provide  for  the  presence 
of  a  number  of  easily  fusible  substances,  although  their  presence 
is  not  always  necessary  for  the  decomposition  of  the  ore.  For 
instance,  even  in  the  assay  of  pure  limestone,  which  is  a  base,  a 
certain  amount  of  sodium  carbonate,  also  a  base,  is  always  added. 

Use  of  Fluxes.  —  For  the  sake  of  economy  in  material  and  time 
it  is  best  to  limit  the  amount  of  fluxes  to  the  needs  of  the  ore. 
The  great  saving  to  be  made  in  this  way  may  be  illustrated  as 
follows:  If  we  use  twice  as  much  flux  as  necessary,  we  have  to 
use  twice  as  large  a  crucible  which  cuts  down  the  furnace  capacity 
very  considerably.  Besides  this,  the  large  charges  require  a 
longer  time  in  the  furnace  to  fuse  and  decompose  the  ore  and  this 
again  reduces  the  furnace  capacity. 

The  Lead  Button.  —  In  every  gold  and  silver  assay,  a  care- 
fully regulated  amount  of  the  litharge  is  reduced.  This  results 
in  the  formation  of  a  great  number  of  minute  globules  of  lead 
which  serve  to  collect  the  gold  and  silver.  When  the  charge 
becomes  thoroughly  liquid  these  collect  in  the  bottom  of  the 
crucible  forming  the  lead  button.  There  is  considerable  differ- 
ence of  opinion  as  to  the  proper  size  for  the  lead  button.  Many 
assayers  hold  that  it  should  be  proportional  to  the  total  volume 
of  charge;  others  vary  the  lead-fall  according  to  the  quantity 
of  precious  metals  to  be  collected.  Both  of  these  ideas  appear 
to  have  merit  and  agree  in  general  with  the  experience  of  lead 


152  A   TEXTBOOK  OF  FIRE  ASSAYING 

blast-furnace  operators,  who  insist  that  the  charge  shall  contain 
not  less  than  10  per  cent  of  lead,  all  of  which,  of  course,  they 
attempt  to  reduce. 

Miller*  and  Fulton,  in  experimenting  on  an  ore  containing 
2260  ounces  of  silver  per  ton,  found  that  the  silver  recovered 
from  the  lead  button  increased  regularly  with  the  increase  in 
size  of  the  lead  button  to  a  maximum  of  28  grams.  They  con- 
cluded that  the  collecting  power  of  a  given  weight  of  lead  was 
independent  of  the  amount  of  the  charge. 

In  most,  cases  a  28-gram  lead  button  will  collect  all  the  gold 
and  silver  in  the  ordinary  crucible  charge,  and  the  assayer  is 
advised  to  figure  for  a  button  of  this  size  unless  some  good  reason 
for  change  is  shown. 

The  Cover.  —  In  practically  all  crucible  assay  work  it  was 
formerly  customary  to  place  a  cover  of  some  fusible  substance 
on  top  of  the  mixed  charge  in  the  crucible.  Different  assayers 
advocate  different  materials,  as  salt,  sodium  sulphate,  borax, 
borax-glass  and  soda,  as  well  as  different  flux  mixtures. 

The  idea  which  leads  to  the  use  of  the  cover  is  that,  melting 
early,  it  makes  a  thick  glaze  on  the  sides  of  the  crucible  above 
the  ore  charge  and  that,  if  particles  of  ore  or  lead  globules  are 
left  on  the  sides  of  the  crucible  by  the  boiling  of  the  charge,  the 
cover  tends  to  prevent  them  from  sticking  there.  As  the  fusion 
becomes  quiet  and  the  temperature  rises,  most  of  this  glaze  runs 
down  to  join  the  main  charge  and  carries  with  it  any  small  parti- 
cles of  ore  or  lead  which  may  have  stuck  to  it  in  the  early  part 
of  the  fusion. 

The  salt  cover  is  thinly  fluid  when  melted.  It  does  not  enter 
the  slag  but  floats  on  top  of  it,  thus  serving  to  keep  out  the  air 
and  to  prevent  loss  by  ebullition. 

The  borax  cover  fuses  before  the  rest  of  the  charge.  It  is 
thick  and  viscous  when  melted  and  serves  to  prevent  loss  of  fine 
ore  by  "  dusting/'  as  well  as  to  stop  loss  by  ebullition.  It  finally 
enters  the  slag  and  so  ceases  to  be  a  cover  after  the  fusion  is  well 
under  way. 

Some  assayers  object  to  the  use  of  salt  on  the  ground  that  it  is 

likely  to  cause  losses  of  gold  and  silver  by  volatilization.     It  is 

a  well-known  fact  that  gold  chloride  is  volatile  at  a  comparatively 

low  temperature,  commencing  at  180°  C.  and  that  silver  chloride 

*  School  of  Mines  Quarterly,  17,  pp.  160-170. 


THE   CRUCIBLE  ASSAY  153 

is  volatile  in  connection  with  the  chlorides  of  arsenic,  antimony, 
copper,  iron,  lead,  etc.  When  an  ore  contains  substances  such 
as  manganese  oxide,  basic  iron  sulphate  etc.,  capable  of  generating 
chlorine  upon  being  heated  with  salt,  it  would  seem  wise  to  omit 
the  use  of  salt.  If  it  is  not  desired  to  use  salt  a  good  cover  may 
be  made  from  a  mixture  of  borax-glass  and  sodium  carbonate 
in  the  proportion  of  10  parts  of  the  former  to  15  parts  of  the 
latter. 

The  present  tendency  is  to  do  away  with  the  cover  altogether. 
For  muffle  fusions,  at  any  rate,  a  salt  cover  is  entirely  unnecessary 
and  even  objectionable,  in  that  it  fills  the  room  with  chloride 
fumes  at  the  time  of  pouring.  Salt  assists  in  the  volatilization 
of  lead  compounds  and  these  are  most  injurious  to  health. 

REDUCTION   AND    OXIDATION. 

Reducing  and  oxidizing  reactions  are  common  in  fire  assaying 
as  in  other  chemical  work,  and  practically  all  fusions  are  either 
reducing  or  oxidizing  in  nature.  For  instance,  the  scorification 
assay  is  an  oxidizing  fusion  in  which  atmospheric  air  is  the 
oxidizing  agent,  while  the  crucible  fusion  of  a  siliceous  ore  is  a 
reducing  fusion  in  which  argols,  flour  or  charcoal  act  as  the  re- 
ducing agents. 

By  the  term  "reducing  power,"  as  used  in  fire  assaying,  is  meant 
the  amount  of  lead  that  1  gram  of  the  ore  or  substance  will  reduce 
when  fused  with  an  excess  of  litharge.  For  instance,  if  we  use 
5.00  grams  of  ore  and  obtain  a  lead  button  weighing  16.50  grams 
the  reducing  power  of  the  ore  is 


By  the  term  "oxidizing  power"  is  meant  the  amount  of  lead 
which  1  gram  of  the  ore  or  substance  will  oxidize  in  a  fusion, 
or  more  exactly  it  is  the  lead  equivalent  of  a  certain  amount  of 
reducing  agent  or  ore  which  is  capable  of  being  oxidized  by  1  gram 
of  the  ore  or  substance. 

Reducing  Reactions.  —  The  reduction  of  lead  by  charcoal  is 
shown  by  the  following  reaction: 

2PbO  +  C  =  2Pb  +  C02, 


154  A   TEXTBOOK  OF  FIRE  ASSAYING 

from  which  it  is  seen  that  1  gram  of  pure  carbon  should  reduce 

2  X  207 
— — —  =  34.5  grams  of  lead.     However,  as  charcoal  is  never  pure 

carbon  the  results  actually  obtained  in  the  laboratory  will  be 
somewhat  less,  usually  from  25  to  30.  All  carbonaceous  mate- 
rials have  more  or  less  reducing  power.  Those  most  commonly 
used  as  reducing  agents  in  assaying  are  charcoal,  R.  P.  ±  27.5; 
argols,  R.  P.  8-  12;  cream  of  tartar,  R.  P.  5.5;  flour,  R.  P.  10-12. 
Besides  carbonaceous  matter  many  other  substances  and  ele- 
ments are  capable  of  reducing  lead  from  its  oxide.  The  most 
important  of  these  are  metallic  iron,  sulphur  and  the  metallic 
sulphides.  The  reduction  of  lead  by  iron  is  shown  by  the  follow- 
ing reaction: 

PbO  +  Fe  =  Pb  +  FeO, 

207 
whence  the  reducing  power  of  iron  is  •-=«-  =  3.70. 

.  "^ 

The  reducing  power  of  sulphur  and  the  metallic  sulphides  will 
vary  according  to  the  amount  of  alkaline  carbonate  present. 
For  instance,  the  reduction  of  lead  by  sulphur  in  the  absence 
of  alkaline  carbonates  is  shown  by  the  following  reaction: 

2PbO  +  S  =  2Pb  -f-  S02. 

The  reducing  power  of  sulphur  under  these  conditions  would  be 
.  2  X  207 


32 


=  12.9. 


In  the  presence  of  sufficient  alkaline  carbonates  the  sulphur  is 
oxidized  to  sulphur  trioxide  which  combines  with  the  alkali  to 
form  sulphate.  The  reaction  is  as  follows: 


3PbO  +  S  +  Na2CO3  =  3Pb  +  NajSO4  +  CO2, 

from  which  we  see  that  the  reducing  power  of  sulphur,  under 
these  conditions,  should  be 

621 


32 


=  19.4. 


In  the  same  way  we  find  that  the  reducing  power  of  the  metallic 
sulphides  varies  according  to  the  amount  of  available  alkaline 
carbonate  present.  For  instance,  in  the  absence  of  alkaline 
carbonates  and  with  a  small  amount  of  silica  to  slag  the  iron 


THE  CRUCIBLE  ASSAY  155 

oxide  and  to  hold  it  in  the  ferrous  condition,  the  following  equa- 
tion expresses  the  reaction  between  iron  pyrite  and  litharge: 

FeS2  +  5PbO  +  SiO2  =  FeSiO3  +  5Pb  +  2SO2. 

This  last  statement  is  not  strictly  true,  as  in  the  entire  absence 
of  alkaline  carbonate  the  reaction  is.  not  quite  complete.  Miller* 
found  that  under  the  above  conditions  the  lead  button  and  slag 
always  contained  sulphides  and  the  actual  results  fell  slightly 
below  those  called  for  by  the  above  equation.  According  to  this 
equation  the  reducing  power  would  be 

5Pb        1035  _ 
FeS2  =:  120   = 

With  an  excess  of  sodium  carbonate  and  in  the  absence  of 
silica,  the  sulphur  is  oxidized  to  trioxide  and  the  iron  to  the  ferric 
condition,  as  shown  by  the  following  equation: 

2FeS2  +  15PbO  +  4Na2CO3  =  Fe2O3  +  15Pb  +  4Na2SO4  +  4C02, 

and  this  gives  a  reducing  power  of  ^Q"  =  12-9. 

With  a  small  amount  of  silica  present  the  iron  may  be  left 
in  the  ferrous  condition,  which  is  much  to  be  preferred.  Then 
the  reaction  becomes: 

2FeS2  +  14PbO  +  4Na2CO3  +  Si02  = 

Fe2Si04  +  14Pb  +  4Na2SO4  +  4C02, 

which  gives  a  reducing  power  of  12.07. 

All  of  the  above  reactions  may  take  place  simultaneously  in  the 
same  fusion,  and  therefore  it  will  be  obvious  that  there  may  be 
obtained  for  pyrite  any  reducing  power  between  8.6  and  12.9, 
according  to  the  amount  of  sodium  carbonate,  litharge  and  silica 
present.  Unfortunately  it  is  somewhat  difficult  to  control  the 
oxidation  of  the  sulphur,  and  this  makes  it  hard  to  obtain  a  lead 
button  of  the  right  size.  What  the  assay er  wants  to  know  is  the 
"  working  reducing  power  "  of  the  ore,  which  always  lies  some- 
where between  the  two  extremes  indicated,  and  this  he  determines 
by  a  preliminary  fusion  with  a  small  quantity  of  ore,  an  excess 
of  litharge  and  a  carefully  regulated  amount  of  soda. 

The  accompanying  table  gives  the  reducing  power  of  some  of 
the  common  sulphides.  The  theoretical  figures  are  computed 
*  Trans.  A.I.M.E.,  34,  p.  395. 


156 


A   TEXTBOOK  OF  FIRE  ASSAYING 


for  sulphur  oxidized  to  both  SO2  and  SO3.  In  the  last  column 
is  given  the  reducing  power  of  the  pure  minerals  using  the  follow- 
ing charge  Na2CO3  5  gms.,  PbO  80  gms.,  SiO2  2  gms.,  ore  to  yield 
an  approximate  25  gram  button. 

TABLE  XX. 

REDUCING  POWER  OF  MINERALS. 


Computed 

Mineral 

Formula 

Actually 
determined 

S  to  SO2 

S  to  SOs 

Galena 

PbS 

2.6 

3.46 

3.41 

Chalcocite 

Cu2S 

3.9 

5.2 

Arsenopyrite 

FeAsS 

5.7 

6.96 

8.181 

Stibnite 

Sb2S3 

5.5 

7.35 

6.75 

Chalcopyrite 
Sphalerite 

CuFeS2 
ZnS 

6.2 
6.37 

8.44 
8.5 

7.85 
7.87 

Pyrrhotite 

Fe7S8 

7.35 

9.9 

10.  OO1 

Pyrite 

FeS2 

8.6 

12.07 

11.05 

1  The  sample  used  probably  contained  pyrite. 

As  is  the  case  with  sulphur  and  the  metallic  sulphides  the 
amount  of  lead  reduced  by  the  carbonaceous  reducing  agents 
also  depends  upon  the  nature  of  the  charge,  particularly  upon 
the  amount  of  silica  present.  Other  things  being  equal,  the 
more  basic  the  charge,  the  greater  the  amount  of  lead  which  will 
be  reduced  by  a  unit  quantity  of  the  reducing  agent.  Thus, 
a  certain  sample  of  argols  showed  a  reducing  power  of  11.04 
when  silica  for  a  sub-silicate  was  added,  10.93  for  a  mono-silicate, 
10.62  for  a  bi-silicate  and  only  9.26  for  a  tri-silicate.  The  rate 
of  fusion  and  the  final  temperature  both  have  a  good  deal  to  do 
with  the  amount  of  this  reduction,  for  the  reason  that  the  silicates 
of  lead  more  acid  than  the  mono-silicate  are  but  little  reduced 
by  carbon  below  1000°  C.  With  a  limited  amount  of  litharge 
present,  part  is  bound  to  be  converted  into  silicate  before  it  can  be 
reduced  by  carbon,  and  naturally  the  greater  the  proportion  of 
silica,  the  larger  the  amount  of  litharge  which  will  combine  and 
thus  be  rendered  unavailable  for  reduction  by  carbon. 

Oxidizing  Reactions.  —  Certain  metals,  notably  iron,  mangan- 
ese, copper,  cobalt,  arsenic  and  antimony,  are  capable  of  existing 


THE  CRUCIBLE  ASSAY  157 

in  two  states  of  oxidation.  When  fused  with  a  reducing  agent 
the  higher  oxides  of  these  metals  are  reduced  to  the  lower  state 
of  oxidation  at  the  expense  of  the  reducing  agent.  Ores  contain- 
ing these  higher  oxides  are  said  to  have  an  oxidizing  power  on 
account  of  this  property  of  using  up  reducing  agent.  For 
convenience  this  oxidizing  power  is  measured  in  terms  of 
lead,  although  the  bulk  of  the  oxidizing  reaction  in  any  assay 
fusion  is  probably  accomplished  against  the  reducing  agent  of  the 
charge. 

For  instance  if  in  an  assay  fusion  containing  silica  we  have 
ferric  oxide,  sufficient  for  a  bi-silicate,  and  carbon,  the  follow- 
ing reaction  takes  place: 

2Fe2O3  +  C  +  4SiO2  =  4FeSiO3  +  CO2, 

from  which  we  find  that  1  gram  of  Fe203  requires  0.037  gram  of 
carbon  to  reduce  it  to  FeO.  Expressed  in  terms  of  lead  the  re- 
lation would  be  as  follows: 

Fe2O3  +  Pb  =  2FeO  +  PbO. 

207 
That  is  to  say  the  oxidizing  power  of  Fe2O3  is  — —  =  1.31. 

Similarly 

MnO2  +  Pb  =  MnO  +  PbO. 

207 

The  oxidizing  power  of  MnO2  is  -==•  =  2.4,  which  means  that 

o7 

each  gram  of  MnO2  present  in  a  fusion  with  litharge  and  a  reducing 
agent  will  prevent  the  reduction  of  2.4  grams  of  lead.  It  is 
easily  seen,  therefore,  that  this  oxidizing  power  of  ores  must  be 
taken  account  of  in  computing  assay  charges.  The  method  of 
determining  the  oxidizing  power  of  ores  will  be  discussed  later. 

In  the  crucible  assay  of  high  sulphide  ores  it  is  frequently 
necessary  to  add  some  oxidizing  agent  to  the  charge  to  prevent 
the  reduction  of  an  inconveniently  large  lead  button.  A  28-gram 
lead  button  is  usually  sufficiently  large  to  act  as  a  collector  of  the 
precious  metals,  and  were  a  larger  button  obtained,  it  would 
entail  an  extra  loss  due  to  scorification,  or  a  prolonged  cupella- 
tion,  as  well  as  consuming  extra  time  in  this  treatment.  When, 
therefore,  the  ore  charge  would  of  itself  reduce  more  than  28 
grams  of  lead  we  ordinarily  add  potassium  nitrate  or  some  other 
oxidizing  agent.  Niter  is  almost  exclusively  used  in  this  country 


158  A   TEXTBOOK  OF  FIRE  ASSAYING 

for  oxidizing.  Its  action  with  carbon  is  shown  by  the  following 
equation : 

4KNO3  +  5C  =  2K2O  +  5CO2  +  2N2, 

from  which  the  theoretical  oxidizing  power  of  niter  expressed  in 
terms  of  lead  is  found  to  be  5.12.  The  theoretical  oxidizing 
power  may  also  be  figured  from  its  reactions  with  sulphur,  or 
any  of  the  metallic  sulphides  and  will  always  give  substantially 
the  same  result  when  the  degree  of  oxidation  of  the  sulphur  is 
kept  the  same  in  the  reducing  and  oxidizing  reactions. 

The  actual  oxidizing  effect  of  niter  is  always  found  to  be  lower 
than  this,  partly  because  the  niter  ordinarily  used  for  this  purpose 
is  not  100  per  cent  KNOa  and  partly  because  in  the  actual  fusion 
some  oxygen  is  likely  to  escape  unused.  This  loss  of  oxygen 
increases  as  the  acidity  of  the  charge  increases.  The  loss  is 
also  probably  influenced  by  the  depth  of  the  charge,  the  rate 
of  fusion  and  the  temperature.  In  the  case  of  actual  assay  fus- 
ions with  sulphides,  the  oxidizing  power  will  be  found  to  vary 
between  3.7  and  4.7,  the  lower  figure  being  approached  when  the 
charge  contains  considerable  silica  and  borax-glass  and  but  little 
litharge,  the  upper  figure  prevailing  when  no  silica  or  borax  is 
used  and  in  the  presence  of  an  excess  of  sodium  carbonate  and 
litharge. 

With  both  the  reducing  power  of  the  sulphides  and  the  oxi- 
dizing power  of  the  niter  varying  with  different  proportions  of 
sodium  carbonate,  litharge,  borax  and  silica,  as  well  as  with  vari- 
ations of  temperature,  the  problem  of  obtaining  a  lead  button 
of  the  right  size  in  niter  assays  is  not  a  simple  one.  The  only 
solution  is  so  to  control  the  conditions  that  the  state  of  oxidation 
of  the  sulphur  in  the  final  assay  shall  be  the  same  as  that  in  the 
reducing  power  fusion.  This  is  the  first  essential;  the  second  is 
to  decide  on  some  slag  of  definite  silicate  degree  and  always  use 
it;  then  the  proportion  of  oxygen  which  escapes  unused  will  be 
nearly  constant  and  the  oxidizing  power  of- the  niter,  once  deter- 
mined, may  be  depended  on  to  remain  constant. 

With  the  type  of  charge  recommended  in  the  latter  part  of  the 
chapter,  the  oxidizing  power  of  niter  will  be  found  to  lie  between 
4.0  and  4.2  and  with  this  minor  variation  but  little  trouble  should 
be  found  in  properly  controlling  the  size  of  the  button. 

Just  as  we  may  obtain  several  reactions  between  any  of  the 


THE  CRUCIBLE  ASSAY  159 

sulphides  and  litharge  according  to  the  degree  of  oxidation  of 
the  sulphur  and  occasionally  also  of  other  constituents  of  the 
mineral,  so  we  may  also  obtain  several  different  reactions 
between  niter  and  the  sulphides.  For  instance,  in  the  absence 
of  alkaline  carbonates  and  in  the  presence  of  silica,  the  sulphur 
can  be  oxidized  only  to  the  dioxide,  and  the  reaction  between 
niter  and  pyrite  would  be  as  follows: 

2KNO3  +  FeS2  +  SiO2  =  K2O.FeO.SiO2  +  2S02  +  N2. 
In  the  presence  of  an  excess  of  alkaline  carbonate  and  litharge 
with  little  or  no  silica,  both  the  iron  and  the  sulphur  would  be 
oxidized  as  highly  as  possible  and  the  following  reaction  would 
result  : 

6KNO3  +  2FeS2  +  Na2CO3  = 

Fe2O3  +  3K2SO4  +  Na2SO4  +  3N2  +  CO2. 

Ferric  oxide  is  a  most  undesirable  component  of  assay  slags 
and  its  formation  must  be  avoided.  To  prevent  the  iron  from 
going  to  the  ferric  condition  enough  silica  should  be  present  to 
hold  and  slag  it  as  ferrous  singulo-silicate.  If  this  is  provided 
the  reaction  then  becomes 

28KNO3  +  10FeS2  +  6Na2CO3  +  5Si02  = 

5Fe2SiO4  +  14K2SO4  +  6Na2SO4  +  14N2  +  6CO2. 

A  slight  oxidizing  effect  may  be  obtained  by  using  red  lead 
in  place  of  litharge,  and  this  is  sometimes  done,  especially  in 
England  and  the  British  colonies.  The  oxidizing  effect  of  red 
lead  is  shown  by  the  following  reaction: 

Pb3O4  +  Pb  =  4PbO. 

207 
The  oxidizing  power  in  terms  of  lead  is  ^=  =  0.30. 


TESTING  REAGENTS. 

Each  new  lot  of  litharge  and  test  lead  should  be  assayed  for 
silver  and  gold  so  that  when  any  is  found  to  be  present  a  proper 
correction  may  be  made.  Different  lots  of  argols,  and  flour  are 
also  found  to  vary  in  reducing  power,  and  their  reducing  powers 
should  also  be  determined. 

The  following  procedure  is  designed,  first,  to  allow  the  student 
to  determine  the  reducing  power  of  flour,  charcoal  or  other  re- 
ducing agents  and  at  the  same  time  to  determine  the  silver  cor- 


160  A   TEXTBOOK  OF  FIRE  ASSAYING 

rection  for  litharge,  and,  second,  to  familiarize  him  with  the 
principal  operations  connected  with  the  crucible  method  of  assay. 

Procedure.  —  Take  two  E  or  F  pot-furnace  crucibles,  or  12  or 
15  gram  muffle  crucibles. 

Weigh  into  them,  in  the  order  given,  the  following: 

No.  1  No.  2 

Sodium  carbonate       5  grams  Sodium  carbonate      5  grams 

Silica  5       "  Silica  5        " 

Litharge  60      "  Litharge  60        " 

Flour  2.50      "  Charcoal  1.00 

Weigh  the  flour  and  charcoal  on  the  pulp  balance  as  exactly 
as  possible,  the  others  on  the  flux  balance.  Mix  thoroughly  with 
the  spatula  by  turning  the  crucible  slowly  with  one  hand  while 
using  the  spatula  with  the  other.  When  finished  tap  the  crucible 
several  times  with  the  handle  of  the  spatula  to  settle  the  charge 
and  to  shake  down  any  material  which  may  have  lodged  on  the 
sides  of  the  crucible  above  the  charge.  Finally  put  on  a  half- 
inch  cover  of  salt. 

Pot-Furnace  Fusion.  —  Have  a  good  bright  fire  in  the  pot- 
furnace  which  should  not,  however,  be  filled  with  coke  more  than 
halfway  to  the  bottom  of  the  flue.  Place  the  crucibles  so  that 
their  tops  shall  not  be  much  above  the  bottom  of  the  flue.  Place 
a  piece  of  cold  coke  directly  under  each  crucible  as  it  is  put  into 
the  furnace.  Cover  the  crucibles  and  pack  coke  around  them, 
being  careful  to  prevent  the  introduction  of  any  coke  or  dust. 
Close  the  top  of  the  furnace,  open  the  draft  if  necessary  and  urge 
the  fire  until  the  charges  begin  to  fuse.  Then  close  the  draft  and 
continue  the  melting  slowly  enough  to  prevent  the  charges  from 
boiling  over.  When  the  charges  have  finished  boiling,  note  the 
time  and  open  the  draft  if  necessary,  to  get  a  yellow  heat  and 
continue  heating  for  ten  minutes. 

Pour  the  fusions  into  the  crucible  mold,  which  has  been  pre- 
viously coated  with  ruddle,  thoroughly  dried  and  warmed.  When 
the  material  is  cold,  a  matter  of  five  or  ten  minutes  for  a  small 
fusion,  break  the  cone  of  lead  from  the  slag  and  hammer  it  into 
a  cube  to  thoroughly  remove  the  slag.  Weigh  the  buttons  on  the 
pulp  balance  to  the  nearest  tenth  of  a  gram  and  record  the  weights, 
and  reducing  powers  in  the  notebook. 

Save  the  lead  buttons  and  cupel  them.     The  beads  should 


THE   CRUCIBLE  ASSAY  161 

contain  all  of  the  gold  and  silver  in  the  60  grams  of  litharge  used. 
Weigh  the  beads  and  part  to  see  if  gold  is  present.  Record 
the  weights  of  the  beads  and  compute  the  correction  for  silver  in 
30  grams  of  litharge. 

Muffle  Fusion.  —  If  the  fusions  are  to  be  made  in  the  muffle 
have  the  muffle  light  red  and  the  fire  under  such  control  that  the 
muffle  can  be  brought  to  a  full  yellow  in  the  course  of  half  an  hour. 
Place  a  row  of  empty  30-gram  crucibles  in  the  front  part  of  the 
muffle  so  as  to  close  the  space  as  completely  as  possible.  These 
serve  to  keep  the  assays  hot  by  reflection  of  heat  and  so  prevent 
loss  of  heat  by  conduction  through  the  door.  See  that  the  muffle 
door  is  tightly  closed  to  prevent  admission  of  air.  Melt  at  suffi- 
ciently low  temperature  to  avoid  violent  boiling  and  then  when 
the  sound  of  bubbling  is  no  longer  heard,  raise  the  temperature  and 
pour  as  in  the  case  of  the  pot-furnace  fusion. 

Notes:  1.  So-called  silver-free  litharge  can  now  be  purchased  but  even 
this  often  carries  traces  of  gold  and  silver. 

2.  In  assaying  samples  of  litharge  low  in  silver  120  to  240  grams  may 
be  required  to  give  a  bead  of  sufficient  size  to  handle  and  weigh. 

3.  It  is  convenient  to  use  litharge  in  multiples  of  30  grams  and  there- 
fore the  silver  correction  is  based  on  30  grams  o£  litharge. 

4.  The  temperature  which  the  muffle  should  have  before  the  crucibles 
are  introduced  depends  upon  the  number  of  charges  which  are  to  be  put  in 
at  one  time.     If  only  one  or  two  the  temperature  should  be  low  to  avoid 
danger  of  boiling  over.     However,  if  the  muffle  is  to  be  filled  with  crucibles 
the  initial  temperature  may  be  higher,  as  the  crucibles  can  be  depended  upon 
to  decidedly  lower  the  temperature. 

5.  Pour  the  fusions  carefully  into  the  center  of  the  "molds  and  do  not 
disturb  until  the  lead  has  had  time  to  solidify. 

The  following  are  the  reducing  powers  of  some  of  the  common 
reducing  agents: 

Charcoal  25-30        Corn-starch  11.5-13 

Argols  8-12        Sugar  14.5 

Flour  10-12         Cream  of  tartar  4.5-6.5 

ASSAY  OF  CLASS  1  ORES.  GOLD  OR  SILVER. 
This  is  the  most  common  class  of  ores  and  as  it  is  also  the  one 
which  presents  the  fewest  difficulties  for  the  assayer,  it  is  con- 
sidered first.  Actually,  ores  with  no  traces  of  sulphides  are 
somewhat  of  a  rarity,  but  the  methods  given  below  may  be  adap- 
ted to  ores  containing  moderate  amounts  of  sulphides  by  simply 
decreasing  the  amount  of  reducing  agent  used. 


162  A   TEXTBOOK  OF  FIRE  ASSAYING 

Slags  for  Class  1  Siliceous  Ores.  —  To  fuse  a  siliceous  ore, 
basic  fluxes  must  be  added,  the  alkaline  carbonates  and  litharge 
being  the  ones  available.     The  bi-silicates  of  soda  and  lead  are 
readily  fusible  and  sufficiently  fluid  for  the  purpose;    therefore, 
the  basic  fluxes  may  be  limited  to  the  amount  necessary  to  form  these 
silicates.   Sodium  carbonate  and  litharge  combine  with  silica  to  form 
bi-silicates  in  proportions  indicated  in  the  following  equations: 
Na2CO3  +  SiO2  =  Na2SiO3  +  C02, 
PbO  +  SiO2  =  PbSiO3. 

From  a  comparison  of  the  molecular  weights  of  the  left-hand 
members  of  these  equations,  it  may  be  determined  that  one  assay- 
ton  of  pure  silica  will  require  either  51.2  grams  of  sodium  carbonate, 
or  108  grams  of  litharge  to  form  a  bi-silicate. 

As  the  mixed  silicate  of  soda  and  lead  is  generally  more  satis- 
factory than  either  one  alone,  it  is  common  to  use  both  of  these 
basic  fluxes  in  every  fusion,  thus  making  a  double,  or  bi-basic 
silicate.  It  is  customary  to  use  at  least  as  much  sodium  carbonate 
as  ore  in  every  assay.  On  this  basis  it  appears  that  approximately 
three-fifths  of  the  silica  is  fluxed  with  soda,  leaving  two-fifths  of  it 
to  be  fluxed  with  litharge.  Taking  these  proportions,  then,  there 
will  be  required  for  one  assay-ton  of  pure  silica  exactly  30.7  grams 
of  sodium  carbonate  and  43.2  grams  of  litharge. 

In  assaying  an  ore  provision  must  also  be  made  for  a  lead  button 
to  act  as  a  collector  of  the  precious  metals.  A  28-gram  button  is 
usually  sufficient.  To  allow  for  this  it  will  be  necessary  to  add 
30  grams  more  of  litharge  and  also  some  reducing  agent,  say  2J 
grams  of  flour  (R.  P.  12). 

The  charge  will  now  stand  as  follows : 

Ore 1       A.  T. 

Sodium  carbonate 30 . 7  grams 

Litharge  for  slag  43  grams     ]  7 

Litharge  for  button  30  grams] 

Flour  (R.  P.  12) 2|    grams 

The  ore  so  far  considered  has  been  an  ideal  one,  pure  silica, 
which  is  rarely  if  ever  found  in  practice.  The  ordinary  siliceous 
ore  almost  invariably  contains  small  amounts  of  iron  oxide,  vari- 
ous silicates  of  alumina,  pyrite  and  other  sulphides,  as  well  as 
occasionally  more  or  less  calcite,  all  of  which  reduce  the  amount 
of  silica  for  which  basic  fluxes  must  be  supplied.  It  is  obvious 
that  for  such  an  ore  it  is  possible  to  make  a  bi-silicate  slag  with  a 


THE   CRUCIBLE  ASSAY 


163 


somewhat  smaller  amount  of  basic  reagents  than  those  in  the  ideal 
charge  shown  above.  It  will  be  advisable  also  to  use  a  small 
amount  of  borax  in  almost  every  fusion,  as  this  helps  both  in 
fluxing  silica  and  in  slagging  the  basic  oxides.  So  that,  by  round- 
ing out  the  above  charge  and  adding  borax,  the  following  practical 
bi-silicate  charges  for  siliceous  ores  are  obtained : 

Ore  i  A.  T.          1  A.  T.  2  A.  T. 

Soda  (Na2CO3)  15  grams     30      grams     60        grams 

5-10       " 
70 


Borax  3-5 

Litharge  50 

Flour  (R.  P.  12)  2  J 


10-15 

110 

2* 


The  larger  the  amount  of  ore  used  the  more  necessary  it  be- 
comes to  keep  down  the  quantity  of  fluxes.  The  following  charges, 
more  acid  than  the  bi-silicate,  are  regularly  used  by  the  author 
for  the  assay  of  siliceous  tailings. 

Ore  1  A.  T.       2  A.  T. 

Soda  (Na2CO3)  30  grams    60  grams 

Borax  3     "  6       " 

Litharge  60     "        90       " 

Flour  for  a         28  gram  30     gram 


5  A.  T. 
150  grams 
15       " 
180       " 
35  gram  lead  button. 


The  results  obtained  with  the  last  mentioned  charges  are  good; 
the  slags,  of  course,  are  more  viscous  than  the  bi-silicate  slags  but 
they  pour  well  even  when  fusions  are  made  in  the  muffle  furnace. 
The  crucibles  are  practically  unattacked  and  if  of  good  quality,  can 
be  used  for  many  such  fusions,  especially  if  care  is  taken  to  cool 
them  slowly. 

The  following  table  gives  the  amounts  of  the  different  common 
basic  reagents  required  to  form  bi-silicates  with  pure  silica.  This 
will  be  found  useful  in  calculating  assay  charges  for  various  quanti- 
ties of  siliceous  ores. 

TABLE  XXI. 
BI-SILICATE  SLAG  FACTORS  No.   1. 


Si02 

Quantity  of  bases  required 

PbO 

Na2C03 

K2CO3 

NaHCO3 

"1  assay-ton 
10  grams 

108  .     gm. 
37.0gm. 

51.2gm. 
17.6gm. 

66.8  gm. 
22.9  gm. 

81.2gm. 
27.9gm. 

164  A   TEXTBOOK  OF  FIRE  ASSAYING 

One  gram  of  FeO  neutralizes  0.84  grams  SiO2  or  requires  1.4 
grams  borax-glass.  One  gram  of  CaCO3  neutralizes  0.60  gram 
SiO2  or  requires  1.0  gram  borax-glass. 

All  assayers  do  not  agree  on  the  use  of  bi-silicate  slags  for 
siliceous  ores,  and  even  if  they  did  agree  they  might  prefer  different 
proportions  of  sodium  carbonate  and  litharge  than  those  men- 
tioned above.  Many  assayers  consider  it  better  to  make  the 
slag  less  acid  than  the  bi-silicate ;  in  fact  there  are  certain  advan- 
tages in  making  what  is  approximately  a  sesqui-silicate.  The 
quantity  of  basic  fluxes  required  for  this  silicate  may  be  deter- 
mined by  increasing  the  figures  found  in  the  last  table  by  one-third. 

Where  a  large  number  of  assays  are  to  be  made  on  ore  of  about 
the  same  character  it  is  neither  necessary  nor  desirable  to  weigh 
out  each  individual  unit  of  flux,  as  this  would  take  too  much  time. 
Instead,  a  flux  mixture  is  made  up  and  then  a  unit  weight  of  this 
mixture  is  weighed  out  for  each  assay,  or  better  still  a  measure  is 
used  which  delivers  the  proper  amount.  There  are  innumerable 
formulas  for  such  mixtures  and  even  for  the  same  ore  many  differ- 
ent mixtures  are  advocated.  A  good  flux  for  the  assay  of  siliceous 
ores  consists  of  3.5  parts  of  sodium  carbonate,  0.5  parts  of  borax 
and  6  parts  of  litharge.  If  an  assayer  uses  100  grams  of  this 
mixture  per  assay-ton  of  quartz  and  reduces  a  28-gram  lead  but- 
ton he  will  have  what  is  approximately  a  bi-silicate  slag.  If  he 
prefers  he  may  use  125  grams  of  flux  which  gives  practically  a 
sesqui-silicate.  The  latter  proportion  is  somewhat  more  popular 
with  assayers,  and  the  student  is  advised  to  try  both.  It  should 
be  noted,  however,  that  half  of  this  quantity  of  flux  will  not  give 
a  sesqui-silicate  with  half  an  assay-ton  of  ore,  unless  at  the  same 
time  the  reducer  is  limited  to  the  amount  required  for  a  14-gram 
lead  button.  This  latter  procedure  is  not  commonly  followed, 
so  that  for  half  an  assay-ton  of  ore  approximately  75  grams  of 
this  flux  should  be  used,  if  a  sesqui-silicate  and  a  button  of  reason- 
able size  are  to  be  obtained. 

Slags  for  Class  1  Basic  Ores.  —  In  the  assay  of  basic  ores  it  is 
necessary  to  add  acid  fluxes,  silica  and  borax  to  obtain  a  fusible 
slag.  Also,  on  account  of  the  fact  that  the  silicates  of  iron, 
manganese,  calcium  and  magnesium  are  by  themselves  infusible, 
or  nearly  so,  at  the  temperature  of  the  assay-furnace,  it  is  custom-, 
ary  to  add  some  soda  and  excess  litharge  to  the  charge.  These 
latter,  combining  with  some  of  the  silica  and  borax,  form  readily 


THE   CRUCIBLE  ASSAY 


165 


fusible  compounds  which  help  to  take  into  solution  the  silicates 
of  the  basic  oxides  and  by  diluting  them  give  more  fusible  and 
fluid  slags.  A  weight  of  soda  equal  at  least  to  that  of  the  ore  is 
generally  taken  as  a  starting  point,  and  very  often  a  quantity  of 
litharge  equal  to  that  of  the  ore  is  also  allowed  for  the  slag. 

The  silicate-degree  of  the  slag  will  depend  on  the  character  of 
the  bases.  For  Class  1  ores,  consisting  principally  of  iron,  man- 
ganese, calcium,  or  magnesium  it  has  been  found  best  to  approxi- 
mate a  sesqui-  or  a  bi-silicate  slag. 

If  the  silica  and  borax  are  cut  down  so  as  to  make  mono-sili- 
cates, the  slags  from  limestone  and  dolomite  will  be  lumpy  when 
hot  and  full  of  lead  shot  when  cold.  Those  from  iron  oxide  will 
be  lumpy  when  hot,  and  when  they  are  poured  the  crucible  will 
be  left  full  of  lead'  shot  which  refuse  to  collect.  When  cold, 
the  slag  will  be  found  full  of  shots  of  lead  and  will  be  magnetic. 
This  is  due  to  the  formation  of  the  magnetic  oxide  of  iron,  which, 
being  infusible,  floats  around  in  the  lower  part  of  the  slag  and 
interferes  with  the  settling  of  the  reduced  lead. 

The  following  table  of  bi-silicate  slag  factors  will  facilitate  the 
calculation  of  charges  for  basic  ores. 

TABLE    XXII. 
BI-SILICATE  SLAG  FACTORS  No.  2. 


Quantity  of  bases 

Quantity  of  arid  required 

1  A.  T.  FeO 

24.5  grams  SiO2 

1  A.  T.  CaCOs 

17.4 

1  A.  T.  MgCO3 

20.8 

t 

10  gms.  PbO 

2.7 

1 

30     "     NaHC03 

10.8 

1 

30    "     Na2CO3 

17.0 

e 

10     "     K2CO3 

4.4 

ft 

For  sesqui-silicates  use  three-quarters  of  the  above  quantities 
of  silica.  When  borax-glass  is  substituted  for  silica,  1  gram  of 
borax-glass  may  be  considered  equivalent  to  0.6  gram  of  silica. 

The  amount  of  silica  which  should  be  replaced  by  borax  has 
not  been  determined,  but  on  account  of  the  greater  fusibility  and 
fluidity  of  boro-silicates  it  is  well  to  replace  at  least  a  quarter  to  a 
third  of  the  silica  with  its  equivalent  of  borax  or  borax-glass. 
When  the  calculated  amount  of  borax-glass  falls  below  5  grams, 
this  quantity  is  generally  used  as  a  minimum. 


166  A   TEXTBOOK  OF  FIRE  ASSAYING 

The  following  example  will  illustrate  the  use  of  the  table. 
Take  1  assay-ton  of  an  ore  consisting  of  50  per  cent  CaCO3  and 
50  per  cent  Si02.  Start  with  30  grams  of  sodium  carbonate  and 
60  grams  of  litharge,  30  for  the  slag  and  30  for  the  lead  button, 
and  plan  for  a  bi-silicate  slag.  Under  these  conditions  the  silica 
requirements  of  the  different  bases  are  as  follows: 

The  CaCO3  of  the  ore  requires  0.5  X  17.4  =    8.7  grams  SiO2 

30  grams  of  soda  require ...17.0       " 

30  grams  of  litharge  require 8.1       "        " 


Total 33.8 

Deducting  the  silica  of  the  ore,  J  A.  T.  =        14 . 6 


Silica  or  equivalent  borax  necessary 19 . 2 

If  two-thirds  of  this  is  put  in  as  silica,  there  remains  19.2  —  12.8 
=  6.4  grams  of  silica,  for  which  we  must  substitute  the  equivalent 

amount  of  borax-glass,  which  is  -r-  X  6.4  =  10.7  grains. 
The  final  charge  stands 

Ore 1  A.  T. 

Sodium  carbonate 30      grams 

Borax-glass 10. 7      " 

Litharge 60          " 

Flour  (R.  P.  12) 2i 

Silica 12.8      " 

This  charge  contains  17.0  grams  of  CaSi03  and  34.6  grams  of 
Na2SiO3,  or  about  twice  as  much  sodium  bi-silicate  as  calcium 
bi-silicate.  Figure  3  shows  that  such  a  combination  will  melt  at  a 
reasonably  low  temperature.  The  lead  silicate  and  the  borax- 
glass  will,  of  course,  reduce  this  melting  temperature  materially. 
Following  the  procedure  outlined  above  it  may  readily  be  de- 
termined that  for  pure  calcium  carbonate  the  charge  shown  below 
should  be  used: 

Ore .'.1  A.  T. 

Sodium  carbonate 30     grams 

Borax-glass 23.6      " 

Litharge 60          " 

Flour 2J 

Silica.  ..28.3      " 


THE  CRUCIBLE  ASSAY 


167 


This  charge  contains  approximately  equal  amounts  of  the 
bi-silicates  of  sodium  and  calcium,  as  well  as  litharge  and  borax- 
glass.  It  fuses  without  difficulty  and  gives  a  glassy  slag  and  a 
good  separation  of  lead. 

Figure  44  gives  at  a  glance  the  quantity  of  reagents  other  than 
flour  required  to  flux  one  assay-ton  of  any  mixture  of  limestone 
and  silica. 


FIG.  44.  —  Quantity  of  fluxes  required  for  1  A.T.  of  any  mixture  of 
limestone  and  silica. 


Magnesium  silicates  are  somewhat  more  difficult  to  fuse  than 
the  corresponding  calcium  silicates;  but  the  same  method  of 
procedure  should  be  followed  for  ores  containing  magnesite  or 
dolomite  as  for  limestone.  Precious  metal  ores  containing  large 
quantities  of  magnesium  carbonate  are  not  likely  to  be  found;  but 
the  assayer  may  have  to  determine  the  quantity  of  silver  contained 
in  a  magnesia  cupel,  and  for  this  bi-silicate  slags  are  the  best. 

Ores  containing  much  calcium  or  magnesium  carbonate  cause 
considerable  boiling  in  the  crucible,  due  to  their  dissociation  into 
oxide  and  carbon  dioxide  at  a  temperature  about  the  same  as  that 
required  to  melt  the  charge.  The  assayer  should  bear  this  in 
mind  in  selecting  a  crucible  for  such  an  ore. 


168  A   TEXTBOOK  OF  FIRE  ASSAYING 

The  charges  for  ores  consisting  mainly  of  iron  or  manganese 
oxides  are  figured  in  the  same  way  as  for  those  containing  calcium 
carbonate.  In  assaying  ores  containing  iron  or  manganese  oxides, 
more  than  the  ordinary  amount  of  reducing  agent  must  be  added 
to  counteract  the  oxidizing  effect  of  these  minerals. 

Slags  for  Alumina.  —  Alumina  is  the  most  difficult  to  flux  of 
any  of  the  common  metal  oxides.  Fortunately,  pure  alumina  is 
never  found  associated  with  gold  and  silver,  and  the  assayer  is  not 
likely  to  encounter  anything  worse  than  ores  containing  a  good 
deal  of  alumina  in  the  form  of  clay.  Pure  china-clay,  or  kaolinite, 
which  has  the  following  composition,  H4Al2Si2O9,  contains  only 
39.5  per  cent  of  alumina.  Ordinary  clays  contain  more  or  less 
quartz  and  other  minerals,  so  that  even  the  above-mentioned  con- 
tent of  alumina  will  not  have  to  be  dealt  with.  Small  amounts 
of  combined  alumina  are  found  in  many  ores  but  these  cause  no 
trouble  in  the  fusion. 

Metallurgists  have  never  entirely  agreed  as  to  the  behavior  of 
alumina  in  slags.  The  work  of  Day,  Shepherd,  Rankin,  Wright, 
Bowen  and  others  has  thrown  much  new  light  on  the  subject  of 
the  constitution  and  thermal  properties  of  the  ternary  system 
Cap  -  A12O3  -  SiO2.  The  melting-point  curve  of  the  CaO  -  SiO2 
series  was  shown  in  Chapter  I.  Figures  45  and  46  give  the  melting- 
point  diagrams*  of  the  A12O3  —  SiO2  and  the  CaO  — A12O3  series 
respectively. 

The  A1203  —  SiO2  curve  is  almost  a  straight  line  between  the 
melting-point  of  silica,  1625°  C.  and  that  of  alumina,  2050°,  the 
silicate  of  lowest  melting-point  being  the  eutectic  containing  87 
•per  cent  of  silica,  which  melts  at  1610°.  This  curve  is  not  at  all 
like  that  of  the  CaO  —  SiO2  series,  as  it  might  be  expected  to  be 
if  alumina  were  a  base.  It  shows  but  one  compound,  Al2O3.SiO2. 

The  CaO  —  A12O3  curve,  on  the  other  hand,  shows  a  number  of 
compounds  and,  what  is  more  important  to  the  metallurgist,  a  very 
decided  reduction  of  melting  temperature  at  about  the  point 
where  the  components  are  of  equal  weight.  The  compound 
5CaO.3Al2O3,  which  contains  47.8  per  cent  of  CaO,  lies  just 
between  two  eutectics,  both  of  which  melt  at  about  1400°  C.  It 
would  seem  from  the  above,  that  alumina  behaves  more  like  an 
acid  than  a  base,  and  it  is  suggested  that  it  be  so  treated. 

Alumina  makes  slags  viscous,  no  matter  how  it  is  treated,  and 
*  Am.  Jour.  Sci.,  39,  pp.  9  and  11. 


THE  CRUCIBLE  ASSAY 


169 


2000 

1900 

I80° 


/000 
£ 
1500 

1400 
1300 


FIQ.  45.  —  Melting  points  of  the  alumina-silica  series. 


\ 

2&00 

_\ 
\ 

\ 

\ 

2400 

\ 

\ 

2300 

\ 

\ 

V 

\ 

2200 

\ 

\ 

\ 

2100 

\ 
\ 

\ 

°^2000 

\                                                      X 
\                                                   '    - 

\                                                / 

£ 

\                                              / 

£1900 

-                        \                                            ' 
\                                         / 

£ 

\ 

^-  1  o  u  U 

\                cs                         ' 

^1700 

\         ^              / 

1600 

\    ^  //^ 

1500 

\  ^  r^    § 

\            /      *-S^          r*v 

1400 

^      ^ 

1300 

^     ,°°     , 

Ca  0  Al2  Oj 

Fia.  46.  —  Melting  points  of  the  lime-alumina  series. 


170  A   TEXTBOOK  OF  FIRE  ASSAYING 

it  should  not  be  allowed  to  exceed  10  or  15  per  cent  of  the  weight 
of  the  slag.  Borax-glass  should  be  increased  as  the  alumina  in  a 
siliceous  ore  increases.  The  addition  of  lime  has  been  found  help- 
ful in  fluxing  alumina,  as  might  be  expected  from  a  study  of  the 
last  curve.  The  following  charge  gives  good  results  with  pure 
china-clay: 

Clay JA.  T. 

Lime  (CaO) 6  grams 

Sodium  carbonate 20     " 

Borax-glass ; 10    " 

Litharge ' 45    " 

Flour  (R.  P.  12) 2J     " 

Silica 12     " 

Cryolite  is  the  best  flux  for  alumina  and  dissolves  it  readily. 
Cryolite  melts  at  about  1000°  and  dissolves  more  than  20  per 
cent  of  its  weight  of  A12O3.  Either  sodium  fluoride  or  fluorite 
may  be  substituted  if  desired.  The  fluorides  are  all  very  liquid 
when  fused  and  because  of  this  property  are  particularly  helpful 
as  fluxes  for  ores  containing  alumina.  The  addition  of  5  or  10 
grams  of  any  of  the  fluorides  will  be  found  beneficial  with  ores 
containing  large  quantities  of  alumina. 

Procedure.  —  Carefully  van  some  of  the  ore,  estimate  and 
record  in  the  notebook  the  amount  and  character  of  each  of  the 
slag-forming  constituents  and  also  of  any  sulphides  present. 
If  the  ore  is  mainly  siliceous  weigh  out  one  of  each  of  the  following 
charges : 

Charge  (a)  Charge  (b) 

Ore  0.5  A.  T.    Ore  0.5  A.  T. 

Sodium  carbonate     30  grams     Sodium  carbonate     15  grams 
Borax  5      "         Borax  5      " 

Litharge  30      "         Litharge  50      " 

Flour  *  Flour  * 

Use  F  pot-furnace  crucibles  or  if  the  work  is  to  be  done  in  the 
muffle  15-  or  20-gram  muffle  crucibles. 

Weigh  out  the  fluxes  and  place  in  the  crucible  in  the  order 
given,  adding  the  ore  and  flour  last  of  all.  Weigh  the  flour  and 
ore  on  the  pulp  balance,  the  others  on  the  flux  balance.  Mix 

*  Enough  combined  with  the  reducing  material  of  the  ore  to  give  a  28- 
gram  button. 


THE  CRUCIBLE   ASSAY  171 

thoroughly  and  if  the  fusion  is  to  be  made  in  the  pot-furnace 
place  a  half-inch  cover  of  salt  or  soda-borax  mixture  on  top. 
Muffle  fusions,  except  those  for  reducing  power,  do  not  require  any 
covers. 

Fuse  at  a  moderate  red  heat  to  avoid  danger  of  the  charge 
boiling  over  and  when  quiet  raise  the  heat  to  a  bright  yellow. 
In  muffle  fusions  the  assayer  must  depend  upon  the  sound  to  tell 
when  the  bubbling  has  ceased.  Allow  fifteen  minutes  of  quiet 
fusion.  Pour  as  usual,  tapping  the  crucible'  gently  against  the 
mold  if  necessary  to  make  sure  of  getting  out  the  last  globules  of 
lead. 

When  the  material  is  cold,  separate  the  lead  buttons  from  the 
slag,  keeping  them  in  order  (a)  (b).  Record  in  the  notebook  the 
character  and  appearance  of  the  slags,  the  ease  or  difficulty  of 
the  separation  of  each  from  the  lead  buttons,  the  appearance  of 
the  lead  buttons  and  their  degree  of  malleability. 

Weigh  the  lead  buttons  on  the  flux  balance  and  cupel  carefully 
to  obtain  feather  crystals  of  litharge.  Weigh  the  silver  beads, 
correct  for  silver  in  the  litharge  used,  part  and  weigh  any  gold 
found  and  finally  report  the  value  of  the  ore  in  ounces  per  ton. 

Both  of  these  charges  should  give  good  results  on  a  siliceous  ore. 
Charge  (a)  is  a  little  less  expensive,  but  charge  (b)  is  more  com- 
monly used,  as  the  slag  contains  two  bases  and  the  excess  litharge 
will  hold  a  moderate  amount  of  impurities  in  solution.  Charge 
(b)  also  gives  a  better  separation  of  lead  button  and  slag  and  has 
the  further  advantage  that  if  the  ore  contains  slightly  more  sul- 
phide than  was  estimated  the  litharge  will  take  care  of  it,  giving 
a  lead  button  free  from  matte.  If  in  charge  (a),  there  is  more 
carbonaceous  reducing  agent  plus  sulphide  mineral  than  the  30 
grams  of  litharge  can  oxidize,  some  of  the  sulphur  will  combine 
with  various  metals  of  the  charge,  principally  lead,  and  form  a 
matte  which  will  appear  immediately  above  the  lead  button. 

Approximately  30  grams  of  litharge  from  each  charge  will  be 
reduced  to  give  the  28-gram  lead  button  and  is  therefore  not 
available  to  combine  with  the  silica.  The  active*  fluxes  are  then 
in  charge  (a),  30  grams  of  soda,  and  5  of  borax,  totaling  approxi- 
mately two  and  a  half  times  the  ore.  In  charge  (b),  the  active 
fluxes  are  15  grams  of  soda,  5  of  borax  and  20  grams  of  litharge, 

*  By  an  active  flux  is  meant  a  flux  which  is  to  appear  in  the  slag  and 
therefore  does  not  include  the  litharge  which  goes  to  form  the  lead  button. 


172  A   TEXTBOOK  OF  FIRE  ASSAYING 

totaling  approximately  three  times  the  ore.  A  very  good  rule  to 
follow  in  making  crucible  charges  is  always  to  use  at  least  two  and 
a  half  times  as  much  active  flux  as  ore. 

Borax  in  the  charge  should  be  increased  as  the  bases  increase. 
For  an  ore  with  10  or  20  per  cent  of  iron  or  manganese  oxide, 
limestone  or  clay,  add  up  to  10  or  15  grams  of  borax  or  5  to  8  grams 
of  borax-glass.  For  ores  containing  larger  amounts  of  bases,  work 
out  a  charge  from  the  data  given  under  the  discussion  of  "slags 
for  basic  ores." 

For  high-grade  ores  and  those  containing  considerable  quan- 
tities of  such  common  impurities  as  oxides  of  tellurium,  copper, 
bismuth,  arsenic,  antimony,  or  nickel,  the  quantity  of  litharge 
must  be  increased  in  proportion  to  the  amount  of  impurity  present. 
Some  idea  as  to  the  quantity  of  litharge  required  may  be  found  in 
the  chapter  on  Special  Methods  of  Assay. 

Notes:  1.  Some  assay ers  prefer  to  omit  the  borax  from  the  charge 
and  use  a  cover  of  crude  borax  or  borax-glass  in  place  of  the  salt.  A  borax 
cover  may  be  used  to  advantage  with  ores  which  "dust"  in  the  crucible, 
as  the  borax  swells  and  melts  early,  tending  to  catch  and  hold  down  the  fine 
particles  of  ore  which  are  projected  upward  from  the  charge. 

2.  The   crucible  should  never  be  more  than  two-thirds  full  when  the 
charge  is  all  in. 

3.  If  a  silver  assay  alone  is  asked  for,  it  is  customary  to  omit  parting 
and  report  the  combined  precious  metals  as  silver. 

-.  4.  In  assaying  for  gold  alone,  if  sufficient  silver  for  parting  is  not  known 
to  be  present,  a  piece  of  proof  silver  should  always  be  added  to  the  crucible 
or  to  the  lead  button  before  cupeling.  If  the  approximate  amount  of  gold  is 
known,  about  eight  times  its  weight  of  silver  should  be  allowed. 

5.  The  slag  should  be  fluid  on  pouring  and  should  be  free  from   lead 
shot.     If  it  is  pasty  or  lumpy,  either  the  fusion  has  not  been  long  enough  to 
thoroughly  decompose  the  ore,  or  the  charge  is  too  basic  and  more  borax 
and  silica  should  be  added.     The  crucible  should  have  a  thin  glaze  of  slag 
and  should  be  but  little  corroded.     It  should  show  no  particles  of  undecom- 
posed  ore  or  "shots"  of  lead.     These  latter  can  best  be  seen  immediately  after 
pouring,  and  the  student  should  make  it  a  point  to  examine  his  crucible 
immediately  after  every  pour.     Neither  the  cover  nor  the  outside  of  the 
crucible  should  show  any  glazing,  as  this  indicates  that  the  fusion  has  boiled 
over.     The  cold  slag  should  be  homogeneous,  as  otherwise  it  indicates  in- 
complete decomposition  of  the  ore.     Glassy  slags  are  usually  preferred  by 
assayers  but  are  not  essential  for  all  ores. 

6.  A  brittle  slag  is  to  be  preferred,   particularly  one  which  separates 
easily  and  completely  from  the  lead  button.     If  too  acid,  particularly  if  too 
much  borax  has  been  used,  the  slag  is  apt  to  be  tough  and  to  adhere  tena- 
ciously to  the  lead  button  so  that  when  separated  some  of  the  lead  comes  off 


THE  CRUCIBLE  ASSAY  173 

with  the  slag.  This  causes  a  great  deal  of  annoyance  and  is  bound  to  result 
in  some  loss  of  alloy.  By  setting  the  mold  in  cold  water  just  after  the  red  has 
disappeared  from  the  slag,  the  latter  may  be  made  more  brittle.  The  water 
must  not  be  allowed  to  enter  the  mold,  which  must  be  handled  carefully  to 
avoid  disturbing  the  still  liquid  lead. 

7.  If  the  button  is  hard  or  brittle  or   weighs  more   than   30  grams  it 
should  be  scorified  before  cupeling.     Hard  buttons  indicate  the  presence  of 
copper,  antimony,   or  nickel.     Brittle  buttons  may  be  due  to  antimony, 
arsenic,  zinc,  sulphur,  litharge  or  may  be  rich  alloys  of  lead  and  the  precious 
metals. 

8.  Examine  carefully  the  line  of  separation  of  the  slag  and  lead.     The 
separation  should  be  clean  with  no  films  of  lead  adhering  to  the  slag.     There 
should  be  no  third  substance  between  the  slag  and  lead,  nor  should  the  sur- 
face of  the  lead  show  any  disposition  to  crumble  when  hammered.     Any 
lead-gray,  brittle  substance  between  the  lead  and  slag  or  attached  to  the  lead 
button  is  probably  matte.     This  indicates  incomplete  decomposition  of  the 
ore,  due  to  incorrect  fluxing  or  too  short  a  time  of  fusion.     If  the  former  is 
the  cause,  decreasing  the  silica  and  increasing  the  soda  and  litharge  will  usu- 
ally prevent  the  formation  of  this  substance  in  a  subsequent  fusion. 

ASSAY   OF   CLASS  2   ORES. 

Ores  of  this  class  containing  only  small  amounts  of  sulphides 
are  assayed  in  exactly  the  same  manner  as  Class  1  ores  but  with 
lesser  amounts  of  flour.  However,  when  sulphides  are  present 
in  such  amounts  as  to  reduce  a  lead  button  too  large  to  cupel  a 
different  method  of  procedure  must  be  followed.  The  most 
important  methods  for  the  assay  of  these  ores  follow: 

1.  SCORIFICATION.  —  This  method  has  already  been  considered. 
It  is  not  well  suited  for  gold  ores  and  fails  for  many  silver 
ores. 

2.  LITHARGE-NITER  METHOD.  — 'The  reducing  power  of  the  ore 
is  first  determined  by  means  of  a  preliminary  assay.     Using  the 
figure  thus  obtained,  the  assayer  adds  a  certain  amount  of  niter 
to  the  regular  fusion  to  oxidize  a  part  of  the  sulphur  of  the  ore, 
thus  preventing  the  reduction  of  too  large  a  lead  button.     This 
is  the  most  common  method  for  the  assay  of  sulphide  ores.     The 
sulphides  are  decomposed  partly  by  litharge  and  partly  by  the 
niter. 

3.  SODA-IRON  METHOD.  —  The  litharge  added  to  the  charge  is 
kept  low  so  that  the  lead  from  it,  plus  that  in  the  ore,  will  yield 
a  button  of  suitable  size  for  cupeling.     The  sulphide  minerals  of 
the  ore  are  decomposed  by  means  of  the  metallic  iron.     This  is  a 
good  method  for  many  ores  and  is  commonly  used. 


174 


A    TEXTBOOK  OF  FIRE  ASSAYING 


4.  ROASTING  METHOD  —  A    carefully  weighed  portion  of  the 
ore  is  roasted  to  eliminate  sulphur,  arsenic,  antimony  etc.,  and  the 
roasted  ore  is  then  assayed  as  a  Class  1  ore. 

5.  COMBINATION  WET-AND-FIRE  METHOD.  —  The  sulphides,  etc. 
of  the  ore  are  oxidized  with  nitric  acid,  the  silver  is  precipitated 
as  chloride  and  combined  with  the  insoluble  residue  containing 
the  gold.     This  is  filtered  off  and  assayed  either  by  scorification 
or  crucible. 

The  Litharge-Niter  Assay. 

With  half-assay-ton  charges  of  ore  in  fusions  containing  an 
excess  of  litharge,  there  may  be  as  much  as  18  per  cent  of 
pyrite  or  proportionately  larger  amounts  of  other  sulphides,  de- 


j>2.0 
& 


1.0 


0        1.0       2.0      3.0      4.0      5.0      6.0      7.0      8.0      9.0      10.0     ILO 

Reducing  Power 

FIG.  47.  —  Quantity  of  flour  or  niter  required  per  0.5  A.T.  of  ore  of 
any  reducing  power. 

pending  on  their  reducing  power,  and  lead  buttons  of  the  right 
size  for  cupellation  may  still  be  obtained  by  cutting  down  or  en- 
tirely eliminating  the  flour  or  other  reducing  agent.  With  ores 
containing  more  than  18  per  cent  of  pyrite  the  lead  buttons  ob- 
tained will  be  too  large,  unless  some  oxidizing  agent  is  added 
to  counteract  this  extra  reducing  effect.  For  this  purpose  potas- 
sium nitrate  is  commonly  used.  Figure  47  shows  the  quantity 
of  flour;  R.  P.  12,  or  of  niter,  O.  P.  4.2,  which  must  be  added 


THE  CRUCIBLE  ASSAY  175 

to  half  an  assay-ton  of  sulphide  ore  of  any  reducing  power  to 
obtain  a  28-gram  lead  button. 

To  perform  an  intelligent  niter  assay  it  is  also  necessary  to 
know  whether  the  ore  is  a  simple  sulphide  of  lead,  iron,  or  zinc 
or  whether  it  contains  considerable  amounts  of  metal  impurities 
such  as  tellurium,  copper,  bismuth,  arsenic,  antimony,  nickel  or 
cobalt.  In  the  latter  case  special  measures  have  to  be  taken  to 
eliminate  these  so-called  "  impurities."  In  the  discussion  of  the 
process  the  simple  case  of  the  assay  of  "  pure  ores  "  will  be  taken 
first. 

Slags  for  Pure  Ores.  —  When  an  ore  contains  so  large  a  pro- 
portion of  sulphide  minerals  that  it  is  necessary  to  add  niter  to 
prevent  the  reduction  of  too  much  lead,  it  will  be  found  that  the 
charges  recommended  for  Class  1  ores  will  not  allow  a  satisfactory 
decomposition  of  the  ore.  Instead  of  two  products,  slag  and  lead, 
a  third  intermediate  product,  matte,  is  often  obtained  as  the  result 
of  the  fusion.  This  amounts  to  an  incomplete  decomposition  of 
the  ore  and  as  matte  is  a  good  collector  of  precious  metals  its 
presence  is  a  sure  indication  of  low  results.  A  matte  is  much  less 
likely  to  be  formed,  however,  with  a  less  acid  charge  and  it  has 
been  found  best,  therefore,  to  make  a  slag  approaching  a  mono- 
silicate  for  all  sulphide  ores,  as  by  this  means  more  uniformly 
satisfactory  results  are  obtained. 

A  moderate  excess  of  litharge  is  always  desirable  in  this  method 
as  it  assists  in  the  oxidation  of  the  sulphides  and  also  tends  to 
keep  the  metal  impurities  out  of  the  lead  button.  For  this  reason 
no  less  than  60  grams  of  litharge  per  half  assay-ton  should  be 
used.  Fifteen  grams  of  sodium  carbonate  should  be  provided 
for  the  slag,  as  well  as  a  small  amount  in  addition,  to  combine 
with  the  SO3  not  taken  care  of  by  the  K2O  of  the  niter. 

In  calculating  a  charge,  the  silica  requirements  of  the  various 
bases  are  determined,  just  as  in  the  case  of  Class  1  basic  ores, 
and  the  silica  in  the  ore  is  deducted.  A  minimum  of  5  grams  of 
borax-glass  is  generally  used;  in  the  case  of  ores  containing  much 
zinc  this  should  be  increased  to  10  grams.  The  silica  equivalent 
of  the  borax-glass  is  deducted  from  the  calculated  amount  of 
silica  required. 

Slags  for  Impure  Ores.  —  When  the  ore  consists  mainly  of 
sulphides  or  nickel,  antimony,  arsenic,  bismuth,  copper  or  tel- 
lurium the  type  of  charge  mentioned  above  does  not  contain 


176  A   TEXTBOOK  OF  FIRE  ASSAYING 

enough  uncombined  litharge  to  keep  the  impurities  out  of  the  lead 
button.  The  remedy  is  to  increase  the  litharge  without  increas- 
ing the  silica,  thus  increasing  the  amount  of  uncombined  litharge 
in  the  slag  and  thereby  having  it  available  for  the  solution  of  the 
base  metal  oxides.  R.  W.  Lodge*  recommends  the  use  of  from 
15  to  25  per  cent  of  litharge  in  excess  of  that  called  for  by  the 
reducing  power  of  the  ore  and  this  yields  satisfactory  results 
with  most  impure  ores.  It  calls  for  an  increase  in  litharge  as 
the  reducing  power  of  the  ore  increases.  In  the  case  of  impure 
ores,  this  is  equivalent  to  an  increase  of  litharge  with  an  in- 
crease of  impurities.  It  is  desirable  in  this  case  to  figure  for 
a  sub-silicate  slag.  More  detailed  instructions  for  the  assay  of 
impure  ores  will  be  found  in  the  following  chapter. 

Disadvantages  of  Excess  Litharge.  —  Owing  to  its  property 
of  dissolving  and  forming  easily  fusible  mixtures  with  oxides 
of  the  metals  which  are  in  themselves  difficultly  fusible,  and  par- 
ticularly because  of  its  property  of  keeping  the  impurities  out  of 
the  lead  button,  litharge  has  become  the  assayers  "  cure-all." 
The  student  should  have  in  mind,  however,  the  possible  disad- 
vantages of  the  use  of  too  much  litharge.  These  include  the  extra 
cost  of  the  added  reagent  and  the  more  rapid  destruction  of  cru- 
cibles, which  most  assayers  prefer  to  use  for  a  number  of  fusions. 
More  important  than  the  latter,  is  the  damage  which  is  done  in 
case  a  crucible  is  eaten  through,  thus  allowing  this  corrosive  slag 
to  run  out  on  the  muffle  floor.  It  has  long  been  recognized,  also, 
that  an  increase  of  litharge  increases  very  slightly  the  quantity  of 
silver  which  is  held  in  the  slag,  so  that  no  more  litharge  than  is 
necessary  to  ensure  a  pure  lead  button  and  proper  decomposition 
of  the  sulphide  should  ever  be  used. 

Conduct  of  the  Fusion.  —  It  was  formerly  believed  that 
charges  containing  niter  require  very  slow  and  careful  heating 
to  prevent  loss  due  to  boiling-over,  and  in  some  quarters  this 
impression  still  prevails.  This  danger  of  loss  due  to  boiling  is  a 
real  one  if  fusions  are  made  in  coke  pot-furnaces,  as  was  formerly 
the  custom;  but  to-day,  in  this  country  at  least,  practically  all 
regular  assay  fusions  are  made  in  large  muffles.  In  the  coke- 
fired  pot-furnace  the  charge  is  unevenly  heated,  the  bottom 
melts  while  the  top  is  still  cold.  Somewhere  between  the  two  is 
a  zone  of  viscous  semi-melted  material  which  tends  to  be  lifted 
*  Notes  on  Assaying,  2nd  Ed.,  p.  105. 


THE   CRUCIBLE   ASSAY  177 

bodily  out  of  the  crucible  by  the  ascending  gases.  In  the  muffle, 
on  the  other  hand,  the  crucibles  are  evenly  heated  from  all  sides 
and  because  of  the  heat-retarding  effect  of  the  bottom  and  sides 
of  the  crucible,  fusion  probably  begins  at  the  top  and  proceeds 
downward.  This  provides  a  fluid  slag  through  which  the  gases 
may  readily  escape,  so  that  the  charge  boils  up  only  very  little. 

For  the  best  results  in  niter  fusions  the  crucibles  should  be 
introduced  into  a  hot  muffle  and  brought  rapidly  to  fusion,  the 
whole  fusion  process  not  taking  more  than  ten  or  fifteen  minutes. 
This  method  of  procedure  ensures  a  complete  decomposition  of 
the  sulphide  minerals  of  the  ore  and  prevents  the  formation  of  a 
matte  which  is  likely  to  result  if  the  fusion  takes  a  long  time. 
The  crucibles  should  be  in  the  furnace  not  more  than  thirty,  or  at 
the  most  forty  minutes.  If  a  number ,  of  crucibles  are  to  be 
charged  at  one  time  the  furnace  should  be  at  a  light  yellow  heat. 
The  cold  crucibles  will  lower  the  temperature  materially  and  it 
need  not  be  heated  above  a  yellow  heat,  about  1000°  C.,  to  finish. 
In  fact  a  higher  finishing  temperature,  particularly  if  maintained 
for  some  time,  will  cause  low  silver  results,  probably  due  to  vol- 
atilization. 

To  obtain  good  results,  particularly  when  a  large  amount  of 
litharge  is  used  in  the  charge,  the  muffle  door  should  be  closed 
tightly  and  a  reducing  atmosphere  maintained  in  the  muffle. 
If  coal-fired  muffles  are  used  for  fusions,  the  holes  in  the  back  of 
the  muffles  should  be  closed  and  several  crucibles  containing 
bituminous  coal  placed  in  the  front  part.  This  latter  precaution  is 
unnecessary  with  gas,  gasoline,  or  oil-fired  furnaces  as  these  ordina- 
rily have  a  reducing  atmosphere  in  the  muffle  or  crucible  chamber. 

The  quick  fusion  which  occurs  in  properly  conducted  niter 
assays  effects  a  rapid  and  apparently  complete  decomposition  of 
the  ore,  but,  except  in  the  most  skilful  hands,  the  slag  losses  are 
higher  than  for  Class  1  ores  of  corresponding  grade.  The  rapid 
fusion  and  very  liquid  slag  do  not  permit  globules  of  lead  to  re- 
main in  suspension  for  more  than  a  few  moments  and  the  high 
slag  losses  common  with  this  method  may  be  due  partly  to  the 
less  complete  collection  of  the  precious  metals  by  the  lead.  For 
this  reason  it  is  essential  to  reduce  a  generous  amount  of  lead  in 
this  assay,  not  less  than  25  grams  and  even  35  grams  in  the  case 
of  large  charges.  The  use  of  the  large  quantity  of  litharge  and 
niter  required  in  the  assay  of  impure  sulphide  ores  is  thought  to 


178  A    TEXTBOOK  OF  FIRE  ASSAYING 

give  high  slag  losses,  due  to  oxidation  of  silver  and  its  solution  in 
the  heavy  litharge  slag. 

Perkins*  finds  that  the  excessive  silver  loss  in  this  kind  of  a 
slag  may  be  largely  prevented  by  maintaining  a  reducing  atmos- 
phere in  the  furnace  throughout  the  fusion  period. 

Physical  and  Chemical  Changes  Taking  Place  in  Niter 
Fusions.  —  When  the  crucibles  are  placed  in  the  furnace  the 
temperature  of  the  charge  immediately  begins  to  rise,  and  soon 
any  hydroscopic  water  contained  in  the  reagents  is  driven  off. 
When  the  temperature  reaches  339°  C.,  at  which  point  niter 
melts,  the  charge  begins  to  frit  and  some  of  the  sulphides  com- 
mence to  react  with  the  niter,  although  the  action  is  slow  at  this 
temperature.  At  about  450°,  silica  begins  to  react  on  the  niter 
with  the  evolution  of  oxygen,  nitrogen  and  the  formation  of  po- 
tassium silicate.  The  oxygen  evolved  reacts  with  some  of  the 
more  readily  oxidized  sulphides,  particularly  pyrite  which  be- 
gins to  oxidize  readily  at  about  this  temperature. 

Borax-glass  begins  to  soften  and  combine  with  litharge  at 
about  500°,  and  the  fritting  of  the  charge  increases.  At  530° 
niter  begins  to  dissociate  and  the  oxygen  evolved  helps  to  roast 
the  still  solid  sulphides  and  probably  converts  some  of  the  litharge 
into  PbsCX,  thus  making  of  it  an  oxygen  carrier.  Any  pyrite 
remaining  begins  to  decompose  at  575°  forming  pyrrhotite  and 
sulphur,  but  this  reaction  is  slow  until  the  temperature  reaches 
665°.  Even  in  the  absence  of  other  fluxes,  litharge  and  silica 
begin  to  combine  at  about  700°  C.  to  750°  and  at  this  tempera- 
ture the  charge  becomes  decidedly  pasty,  particularly  in  the 
presence  of  sodium  carbonate  and  borax.  If  the  temperature 
were  to  be  held  at  this  point  the  charge  might  boil  over  on  account 
of  its  pasty  consistency,  but  the  properly  conducted  fusion  is 
heated  rapidly  to  900°  or  1000°  C.,  and  at  this  temperature  it 
is  entirely  fluid  and  bubbles  escape  freely.  The  rate  of  oxidation 
of  the  sulphides  increases  rapidly  as  the  temperature  rises  and 
these  reactions  evolve  a  large  amount  of  heat. 

At  about  750°  some  of  the  metallic  oxides  and  sulphates  begin 
to  react  with  the  undecomposed  sulphides  and  these  reactions 
are  endothermic.  The  following  are  examples: 

PbS  +  PbSO4  =  2Pb  +  2S02  -  92,380  caL, 
PbS  +  2PbO  =  3Pb  +  SO2  -  52,540  cal. 
*  Trans.  A.I.M.E.,  33,  p.  672. 


THE   CRUCIBLE  ASSAY  179 

The  last  reaction  is  only  one  of  many  similar  ones,  which  might 
be  written,  showing  the  direct  reduction  of  sulphides  by  litharge. 
In  the  presence  of  sufficient  silica,  any  ferric  oxide  which  is  present 
will  be  reduced  to  the  ferrous  state  by  sulphides  with  the  liber- 
ation of  sulphur  dioxide,  as  for  example : 

3Fe2O3  +  FeS  +  xSiO2  =  7FeO.xSiO2  +  SO2  -  81,640  cal. 

This  reaction  is  of  importance  above  900°. 

In  this  assay  the  niter  is  limited  to  less  than  that  required  to 
entirely  decompose  the  sulphides  of  ore,  the  amount  of  unde- 
composed  sulphide  left  being  just  enough  to  react  with  litharge 
and  give  a  lead  button  of  the  right  size  for  cupellation.  No  one 
knows  in  just  what  order  the  reactions  take  place,  but  the  net 
result  is  the  same  as  if  the  niter  continued  to  react  until  entirely 
consumed  and  then  the  remaining  sulphide  was  oxidized  by  lith- 
arge. 

It  is  noteworthy  that  all  authorities  recommend  the  use  of  an 
excess  of  litharge  for  the  niter  assay,  although  it  may  be  recalled 
that  it  is  possible  to  decompose  the '  sulphides  entirely  by  fusion 
with  sodium  carbonate  and  niter  alone,  as  in  the  Fresenius  method 
for  the  determination  of  sulphur  in  pyrite.  This  brings  up  the 
question,  "  Why,  and  how  much,  excess  litharge  is  needed?  " 
Beringer*  answers  the  first  half  of  the  question  by  explaining,  as 
is  now  well  known  to  all  assay ers,  that  "  when  metallic  sulphides 
are  present  in  the  ore,  an  excess  of  oxide  of  lead  helps  to  keep  the 
sulphur  out  of  the  button  of  metal,"  in  other  words,  helps  to 
prevent  the  formation  of  a  matte.  Lodgef  calls  for  an  excess 
above  that  required  for  the  reducing  power  of  the  ore,  but  this  is 
only  necessary  with  impure  ores  when  the  litharge  is  required 
to  hold  these  impurities  in  the  slag. 

It  is  obvious  that  every  reagent  has  some  influence  on  the 
result,  but  with  enough  litharge  to  provide  a  lead  button  and  some 
small  excess  to  help  in  making  a  fusible  slag,  the  quantity  of 
silica  present  and  the  rate  of  fusion  have  the  greatest  effect  on 
the  result.  The  presence  of  too  much  silica  in  proportion  to  the 
bases,  or  too  slow  a  fusion,  will  result  in  the  formation  of  a  matte, 
and  this  means  incomplete  decomposition  of  the  ore.  The  reason 
for  this  is  not  difficult  to  find.  In  the  slow  fusion  at  a  low  tem- 

*  A  Textbook  of  Assaying,  13th  Ed.,  p.  93. 
t  Notes  on  Assaying.  2nd  Ed.,  p.  105. 


180  A    TEXTBOOK  OF  FIRE  ASSAYING 

perature  in  the  presence  of  an  excess  of  silica,  the  litharge  will  be 
entirely  converted  into  silicate  before  the  completion  of  the  reac- 
tions resulting  in  the  oxidation  of  the  sulphur,  which  require  a 
comparatively  high  temperature.  The  litharge  contained  in  the 
lead  silicate  is  no  longer  available  for  the  decomposition  of  the 
sulphides,  the  niter  is  all  used  up  and  hence  sulphide  sulphur 
remains  and  a  matte  results.  The  slower  the  fusion,  the  more 
excess  litharge  there  must  be,  and  the  more  basic  the  slag  must  be, 
to  ensure  the  presence  of  enough  undecomposed  litharge  to  com- 
plete the  oxidation  of  the  residual  sulphides.  With  only  enough 
silica  present  for  a  sub-silicate,  the  fusion  may  be  relatively  slow 
and  yet  afford  complete  decomposition  of  the  ore.  This  type  of 
slag  is  destructive  of  crucibles  and  for  this  reason  it  is  better  to 
use  a  more  acid  slag  whenever  possible.  It  would  be  unwise, 
however,  to  make  a  slag  much  more  acid  than  the  mono-silicate, 
for  the  mono-silicate  of  lead  is  only  partly  reduced  by  metallic 
sulphides  at  the  highest  temperature  of  the  assay-furnace.  With 
this  silicate  degree,  however,  rapid  fusions  are  found  to  result  in 
complete  decomposition  of  the  ore. 

The  silica  which  is  added  in  the  assay  of  high  sulphide  ores 
helps  to  slag  the  metallic  oxides  which  are  derived  from  the  oxi- 
dation of  the  sulphides;  it  helps  to  keep  the  iron  in  the  ferrous 
condition  and  it  serves  to  protect  the  crucibles.  If  possible, 
no  reaction  between  it  and  the  litharge  of  the  charge  should  be 
permitted  until  all  the  niter  has  been  consumed  and  the'remain- 
ing  sulphide  has  been  decomposed  by  litharge.  It  is  impossible 
to  realize  this  ideal  entirely  but  it  may  be  approached  by  using 
comparatively  coarse  silica,  perhaps  40-  or  60-mesh,  so  that  an 
appreciable  time  will  be  required  for  its  complete  solution  in  the 
slag. 

Preliminary  Fusion.  Procedure.  —  Van  some  of  the  ore  and 
estimate  the  character  and  amount  of  the  different  sulphides 
present,  as  well  as  the  amount  and  character  of  the  slag-forming 
constituents.  Take  from  3  to  10  grams  of  ore  according  to  the 
amount  of  sulphide  present,  3  grams  for  pure  pyrite,  and 
correspondingly  greater  amounts  for  ores  containing  less  sul- 
phides. If  the  ore  is  mostly  galena  as  much  as  10  grams  may  be 
taken,  the  idea  being  always  to  get  a  button  of  about  30  grams. 
(See  "  Reducing  Power  of  Minerals.")  Take  twice  as  much 
sodium  carbonate  as  ore,  60  grams  of  litharge  and  up  to  5  grams  of 


THE  CRUCIBLE  ASSAY  181 

silica.  If  the  ore  contains  silica  a  proportionately  smaller  amount 
should  be  added.  Use  an  E  crucible  for  the  pot-furnace  or  a 
12-  or  15-gram  crucible  for  the  muffle.  Weigh  out  the  fluxes 
first,  in  the  order  given  and  place  the  ore  on  top,  mixing  thoroughly 
with  a  spatula.  Place  a  half-inch  cover  of  salt  on  top. 

Fuse  for  ten  or  fifteen  minutes,  finishing  at  a  good  yellow  heat. 
Pour  into  crucible  mold,  allow  to  cool,  separate  the  lead  from  the 
slag  and  weigh  on  the  pulp  balance  to  tenths  of  grams.  Divide 
the  weight  of  the  lead  by  the  weight  of  the  ore  taken. 

It  should  be  noted  that  this  reducing  power  is  not  an  absolute 
thing  but  depends  upon  many  factors,  such  as  the  ratio  of  sodium 
carbonate  to  ore,  the  amount  of  borax,  litharge  and  silica  added,  as 
well  as  the  temperature  at  which  the  fusions  are  conducted. 
Reducing  power  fusions  made  in  the  soft-coal  muffle  furnace  are 
likely  to  give  low  results  on  account  of  oxidation  of  the  sulphides 
and  reduced  lead  during  the  fusion. 

Estimating  the  Reducing  Power  of  Ores. —  In  many  in- 
stances it  is  possible  to  estimate  the  reducing  power  of  an  ore 
within  close  limits.  This  requires  a  knowledge  of  the  reducing 
powers  of  the  common  sulphide  minerals  as  well  as  the  knack  of 
vanning.  The  ore  is  vanned  and  the  per  cent  of  the  various  sul- 
phides estimated.  From  these  data  the  reducing  power  is  found. 
For  instance,  if  the  ore  is  50  per  cent  pyrite  and  the  rest  gangue, 
the  reducing  power  will  be  about  5.5,  50  per  cent  of  R.  P.  of  pure 
pyrite.  If  it  is  40  per  cent  galena  and  10  per  cent  sphalerite,  the 
reducing  power  will  be  40  per  cent  of  3.4  +  10  per  cent  of  7.9  = 
2.15  approximately.  The  reducing  power  of  the  ore  is  equal  to 
the  sum  of  the  products  of  the  reducing  powers  of  the  different 
constituents,  multiplied  by  the  percentage  of  each  in  the  ore, 
divided  by  100.  For  example,  with  an  ore  having  three  constitu- 
ents, A,  B,  and  C,  whose  reducing  powers  are  respectively,  a,  b, 
and  c  and  which  are  present  in  the  ore  to  the  extent  of  x,  y,  and  z 
per  cent  respectively,  the  reducing  power  of  the  ore  would  be 
ax  -f-  by  -f-  cz 
100 

In  general  if  the  amount  of  sulphides  in  the  ore  is  comparatively 
small  and  especially  if  only  0.5  assay-ton  of  ore  is  used,  it  is  a  very 
simple  matter  to  obtain  a  lead  button  of  suitable  size  for  cupeling, 
by  this  means.  If,  for  example,  a  mixture  of  galena  and  gangue 
mineral  contains  50  per  cent  of  galena  the  reducing  power  of  the 


182  A   TEXTBOOK  OF  FIRE  ASSAYING 

•  3  40 

ore  will  be"-^-  =  1.70.     Half   an    assay-ton  of  this  ore  would 

give  a  lead  button  weighing  24.8  grams  without  either  flour  or 
niter.  If  the  galena  had  been  estimated  at  40  per  cent,  half  a 
gram  of  flour  (R.  P.  12)  would  have  been  added  and  the  result 
would  have  been  a  30.8-gram  button  which  could  still  be  cupeled. 
In  a  similar  manner,  if  the  galena  had  been  estimated  at  60  per 
cent  about  1  gram  of  niter  would  have  been  added  and  the  result- 
ing button  of  about  20.8  grams  could  also  have  been  cupeled. 

Again,  in  dealing  with  practically  pure  sulphides,  as  in  the  case 
of  pyrite  or  galena  concentrates,  it  is  easy  to  estimate  the  re- 
ducing power  and  properly  control  the  size  of  the  lead  button. 
Determining  the  Oxidizing  Power  of  Niter.  —  The  oxidizing 
power  of  niter  is  found  by  fusing  a  weighed  amount  with  an 
ore  whose  reducing  power  is  known.  To  obtain  comparative 
results  the  slags  must  be  exactly  like  those  used  for  the  reducing- 
power  fusion  and,  moreover,  to  give  lead  buttons  of  the  proper 
size  in  the  final  assay,  the  slag  that  is  made  there  must  be  similar 
as  regards  acidity,  litharge  excess,  etc.  to  that  made  in  the  prelimi- 
nary fusion. 

The  following  example  illustrates  the  method  of  finding  the 
oxidizing  power  of  niter: 

Ore  5  grams  5  grams 

Sodium  carbonate  10      "  10      " 

Litharge  60      "  60      " 

Niter  4      " 

Silica  5      "  5      " 


Lead  obtained  24.31  grams  6.61  grams 

Reducing  power  of  ore      '      =  4.86 

o 

Lead  oxidized  by  4  grams  of  niter  24.31  -  6.61  =  17.70 
Oxidizing  power  of  niter  =  — ^ —  =  4.42 

Quantity    of   Sodium   Carbonate   Converted   to   Sulphate. — 

When  the  reducing  power  of  the  ore,  its  character  and  the  oxi- 
dizing power  of  niter  are  known  the  charge  for  the  regular  assay 
can  be  made  up.  Assume  that  it  is  desired  to  make  a  slag  con- 
taining 15  grams  of  sodium  carbonate  and  30  grams  of  litharge 
for  0.5  assay-ton  of  ore,  and  that  enough  silica  should  always  be 


THE  CRUCIBLE  ASSAY  183 

present  to  hold  and  slag  the  iron  as  ferrous  singulo-silicate,  thus 
preventing  it  from  becoming  converted  to  ferric  oxide.  With 
pure  pyrite,  the  reducing  power  of  which  may  be  assumed  to  be 
12,  and  with  the  further  assumption  that  all  of  its  sulphur  is  oxi- 
dized to  SO3,  it  is  evident  that  if  the  soda  in  the  slag  is  to  be  kept 
constant  the  soda  which  is  added  to  the  charge  will  have  to  be 
increased  as  the  reducing  power  of  the  ore  increases,  because  one 
of  the  products  of  the  reaction  of  niter  upon  sulphides  in  the 
presence  of  soda  is  sulphate  of  soda,  and  because  the  soda  thus 
used  up  no  longer  serves  as  a  flux. 

The  reactions  governing  the  decomposition  of  the  pyrite  under 
the  assumed  conditions  are  the  following: 

l 

(a)   2FeS2  +  14PbO+  4Na2CO3  +  SiO2  = 

F2SiO4  +  14Pb  +  4Na2SO4  +  4CO2, 

(6)    10FeS2  +  28KNO3  +  6Na2C03,  +  5SiO2  = 

5Fe2SiO4  +  14K2SO4  +  6Na2SO4  +  14N2  +  6CO2. 


28 
When  the  ore  has  a  reducing  power  of  12,  —  =  2.33  grams  of 

pyrite  react  according  to  equation  (a),  yielding  a  28-gram  lead 
button.  From  the  proportion 

FeS2  :  2Na2CO3  =  120  :  212  =  2.33  :  4.12, 

it  may  be  seen  that  this  reaction  results  in  the  conversion  of  4.12 
grams  of  Na2CO3  into  sulphate. 

Reaction  (a)  is  actually  the  last  to  take  place,  but  was  con- 
sidered first  to  determine  the  quantity  of  pyrite  in  excess  of  that 
required  to  furnish  the  lead  button,  as  this  is  the  amount  which 
must  be  oxidized  by  niter.  There  remains  to  be  decomposed 
by  niter  under  the  conditions  of  equation  (6)  14.58  —  2.33  grams 
=  12.25  grams  of  pyrite.  The  sodium  carbonate  required  to 
satisfy  this  reaction  may  be  found  from  the  proportion 

5FeS2  :  3Na2CO3  =  620  :  318  =  12.25  :  y, 

solving,  y  is  found  to  be  6.28.  Adding  these  two  quantities,  it 
will  be  seen  that  0.5  assay-ton  of  pyrite,  under  these  conditions, 
causes  the  removal  of  10.4  grams  of  Na2CO3  from  the  slag. 

It  is  possible  to  generalize  from  these  figures  and  say  that  each 
gram  of  pyrite  in  the  charge,  up  to  2.33  grams,  requires  the  addition 


184  A   TEXTBOOK  OF  FIRE  ASSAYING 

of  1.75  grams  of  soda-ash,  and  every  gram  of  pyrite  above  2.33 
grams  requires  0.52  grams  of  soda-ash.  The  computations  for 
the  actual  charges  need  not  be  carried  out  in  such  detail,  but  it  is 
done  here  to  illustrate  the  principle. 

The  potassium  and  sodium  sulphates  formed  by  these  reactions 
are  only  slightly  soluble  in  silicate  slags  and,  being  lighter  than 
the  slag,  form  a  layer  on  top  of  it.  This  sulphate  cover  is  very 
liquid  when  molten  and  serves  to  keep  the  air  away  from  the  fusion. 
In  the  mold  it  appears  on  top  of  the  slag  cone  as  a  crystalline  white 
layer. 

Quantity  of  Niter  Required.  —  The  quantity  of  niter  required 
for  any  charge  is  determined  by  multiplying  the  reducing  power 
of  the  ore  by  the  quantity  of  ore  taken  for  assay,  which  gives 
as  a  result  the  quantity  of  lead  which  would  be  reduced  from 
an  excess  of  litharge  if  the  latter  were  present  and  no  niter  were 
added.  From  this  quantity  is  subtracted  the  weight  of  the  lead 
button  desired  and  the  remainder  is  divided  by  the  oxidizing  power 
of  niter,  expressed  in  terms  of  lead.  For  instance,  in  the  case 
referred  to  above,  the  reducing  power  of  pure  pyrite  being  as- 
sumed to  be  12,  the  oxidizing  power  of  niter  in  this  type  of  charge 
to  be  4.2,  the  quantity  of  niter  required  for  0.5  assay-ton  of  ore  is 
determined  as  follows : 

Total  reducing  effect  of  ore  14.58  X  12.0  =  175.0  grams  of  lead 
Lead  button  desired  28.0  " 


Difference,  pyrite  equivalent  of  which  must 

be  oxidized  by  niter  147 .0  " 

Niter  required     .  '     =  35 . 0  grams. 

4.<£ 

This  is  a  large  amount  of  niter  for  0.5  assay-ton  of  ore  and 
considerably  more  than  would  actually  be  required.  The  writer 
has  never  found  an  ore  requiring  more  than  25  grams  of  niter 
for  0.5  assay-ton. 

Silica  Requirements  of  Bases.  —  For  ores  which  consist  of 
the  sulphides  of  iron,  lead  and  zinc,  together  with  gangue  minerals, 
and  which  are  here  classified  as  pure  ores,  a  singulo-silicate  sla£ 
will  give  satisfactory  results.  The  silica  requirements  for  the 
different  bases  entering  the  charge  in  the  example  taken  would 
be  as  follows: 


THE  CRUCIBLE  ASSAY  185 

For  8.65  grams  of  FeO  resulting  from  the 

oxidation    of    0.5    assay-ton    of    pyrite 

there  is  required 3 . 64  grams  8162 

For  15  grams  of  sodium  carbonate  in  the 

slag 4.25       " 

For  30  grams  of  litharge 4.06       "        " 

Total 11.95       " 

Completed  Charge.  —  Combining  these  various  quantities  the 
charge  for  purepyrita  is  found  to  be  as  follows : 

Ore  (Pyrite  R.  p7l2) 0.5    A.  T. 

Sodium  carbonate 25        grams 

Litharge 60  " 

Niter 35l  " 

Silica 11.95      " 

1  About  10  grams  more  than  would  ever  be  required. 

With  the  proper  furnace  treatment  this  charge  will  give  a  good 
decomposition  of  the  ore  with  a  clean  lead  button  and  greenish- 
black,  glassy  slag. 

Most  assayers  would,  however,  add  a  minimum  of  5  grams  of 
borax-glass.  If  this  is  done  the  equivalent  amount  of  silica 
should  be  omitted,  and  the  charge  would  be 

Ore 0.5  A.  T. 

Sodium  carbonate 25     grams 

Borax-glass 5 

Litharge 60 

Niter Q.  S. 

Silica 8        " 

Zinc  oxide  is  difficult  to  slag,  and  the  zinc  silicates  fuse  only 
at  a  very  high  temperature.  Stein*  states  that  ZnSiO3  melts 
at  1479°  C.,  and  Zn2SiO4  at  1880°  C.  In  the  presence  of  much 
zinc  the  borax-glass  may  be  increased  to  a  maximum  of  10  grams 
in  case  of  pure  sphalerite.  It  is  interesting  to  note  that  the  addi- 
tions of  borax  increases  the  solubility  of  the  sulphate  salt  in  the* 
slag.  With  pure  sphalerite  and  no  borax  the  slag  is  glassy  and 
the  weight  of  the  sulphate  cover  closely  approaches  the  theo- 
retical amount.  When  10  grams  of  borax-glass  is  added  the 

*  Zeitschr.  anorg.  Chemie,  55,  p.  179  (1907). 


186 


A   TEXTBOOK  OF  FIRE  ASSAYING 


No.  2 

No.  3 

Pure  Sphalerite 

Pure  Pyrite 

R.  P.  8.5 

R.  P.  12.0 

0.5A.T. 

0.5  A.  T. 

21      grams 

25      grams 

10 

5 

60 

60 

23 

35 

6 

8 

25-gram 

30-gram 

•  solid  slag  appears  slightly  stony  and  a  much  smaller  sulphate 
cover  is  obtained.  The  alkaline  sulphates  are  dissolved  in  the 
superheated  slag  but  tend  to  crystallize  out  on  cooling,  resulting 
in  the  stony  appearance  of  the  solid  slag. 

The  following  are  examples  of  suitable  charges  for  pure  ores: 


No.  1 

Pure  Galena 
R.  P.  3.45 

Ore 0.5  A.  T. 

Sodium  carbonate .  19      grams 

Borax-glass 0  " 

Litharge 50          " 

Niter  (O.  P.  4.2) .  .   5 

Silica 5 

Crucible 20-gram 


Procedure.  Regular  Niter  Fusion.  —  Make  up  charges  ac- 
cording to  the  rules  outlined  above.  The  fusion  should  pre- 
ferably be  made  in  large  muffle  furnaces  regulated  so  as  to 
have  a  slight  reducing  atmosphere.  It  is  best  to  close  the  holes 
in  the  back  of  soft-coal  muffles  with  bone-ash,  but  this  precaution 
is  unnecessary  with  gas  or  oil-fired  furnaces.  Crucible-type 
furnaces  heated  by  gas  or  gasoline  are  satisfactory  if  thoroughly 
preheated  before  the  crucibles  are  introduced.  Coke-fired  cru- 
cible furnaces  are  the  least  satisfactory  of  all  for  niter  fusions, 
because  of  the  difficulty  of  careful  temperature  control,  which  is 
particularly  necessary  with  this  method.  A  salt  cover  is  entirely 
unnecessary  for  muffle  fusions. 

Be  sure  that  none  of  the  reagents  are  lumpy  and  that  the  charges 
are  thoroughly  mixed.  If  this  precaution  is  taken  and  the  tem- 
perature is  properly  adjusted  no  trouble  should  be  caused  by  boil- 
ing. However,  if  the  soda  and  the  niter  are  in  lumps  the  results 
will  be  less  satisfactory  and  the  charges  may  boil  over.  Fuse  at  a 
high  temperature  so  that  the  charges  will  be  well  melted  in  ten 
minutes  and  will  have  finished  bubbling  in  from  fifteen  to  twenty- 
five  minutes.  After  audible  bubbling  has  ceased  allow  to  remain 
at  a  yellow  heat  for  ten  or  fifteen  minutes  more.  Then  pour  and 
finish  as  usual.  Examine  the  crucibles  while  hot  to  see  whether 
the  fusion  has  been  satisfactory  and  note  particularly  whether 


THE  CRUCIBLE  ASSAY  187 

any  lead  shots  have  remained  behind.  Examine  the  button  and 
line  of  separation  between  lead  and  slag,  to  be  sure  that  lead 
buttons  are  free  from  matte.  If  matte  or  shotty  lead  is  ob- 
tained, the  assay  should  be  repeated  with  such  changes  in  manip- 
ulation of  fire  or  of  composition  of  charge  as  may  be  suggested. 

An  annoying  situation  occasionally  encountered  in  assaying 
some  sulphide  ores,  particularly  those  containing  pyrrhotite  and 
arsenical  pyrite,  is  the  behavior  of  the  lead  which  refuses  to  col- 
lect and  remains  shotted  throughout  the  slag.  When  the  slag  is 
poured,  some  clear  slag  comes  first,  then  slag  full  of  lead  shot. 
The  slag  which  is  left  in  the  crucible  is  also  full  of  lead  shot. 
This  is  usually  due  to  too  low  a  temperature  of  fusion  or  too  little 
silica,  but  may  also  be  caused  by  the  oxidation  of  the  iron  to  ferric 
oxide  during  the  fusion.  Ferric  oxide  is  infusible  at  the  tempera- 
ture of  the  fusion  and  is  insoluble  in  the  ordinary  slag  at  this  tem- 
perature. 

The  best  way  of  overcoming  this  difficulty  is  to  increase  the 
silica  and  run  the  new  assay  at  a  higher  temperature.  If  sufficient 
silica  is  present  to  form  bi-silicates  with  all  of  the  bases,  the  iron 
will  be  held  firmly  in  the  ferrous  condition  and  shots  due  to  this 
cause  are  avoided.  A  high  temperature  favors  the  reduction  of 
Fe2O3  to  FeO  or  what  amounts  to  the  same  thing,  prevents  the 
formation  of  Fe2O3  by  the  niter  and  litharge.  This  is  in  accord- 
ance with  the  well-known  principle  of  physical  chemistry,  that 
"  the  change  of  heat  energy  into  chemical  energy  takes  place  more 
readily  at  high  than  at  low  temperatures."  According  to  data 
given  by  Richards*  the  thermal  equations  representing  this  type 
of  reaction  may  be  written  as  follows: 

Fe2O3  +  Pb  =  2FeO  +  PbO  -  13,400  cal. 
Fe3O4  +  Pb  =  3FeO  +  PbO  -  22,900  cal. 

According  to  van't  HofFs  law,  when  the  temperature  of  such 
a  system  is  raised,  the  equilibrium  point  is  displaced  in  the  direc- 
tion which  absorbs  heat,  that  is  to  say,  the  above  reactions  will 
proceed  in  a  right-handed  direction. 

Ferric  oxide  is  'soluble  in  an  excess  of  litharge,  and  another  way 
to  avoid  obtaining  a  slag  containing  lead  shots  is  to  use  a  large 
excess  of  litharge  in  the  charge.  This  method  of  procedure  is 

*  Metallurgical  Calculations. 


188 


A    TEXTBOOK  OF  FIRE  ASSAYING 


open  to  the  objection  that  the  recovery  of  silver  and  gold  is  more 
or  less  incomplete  when  the  slag  contains  ferric  oxide.* 

The  following  table  of  mono-silicate  slag  factors  may  be  found 
useful  in  determining  the  quantity  of  silica  required  for  any  niter 
fusion. 

TABLE  XXIII. 

MONO-SILICATE  SLAG  FACTORS. 


Quantity  of  bases 

Quantity  of  acids  required 

Silica 

Borax-glass 

8.  65  grams  FeO 
11.3        "      FeO 
12.17      "      ZnO 
13.6        "      PbO 
15  grams  Na2CO3 
30  grams  PbO 

from  £  A.  T. 
"    \  A.  T. 
"    *  A.  T. 
"    |A.  T. 

FeSg  requ  res.  .  . 
Fe7S8          '    .... 
ZnS            <    .... 

PbS           '    .... 

i 

3.64 
4.75 
4.51 
1.84 
4.25 
4.06 

4.89 
6.38 
6.06 
2.44 
5.72 
5.46 

i 

To  avoid  low  results  due  to  oxidation  by  niter,  it  is  often  ad- 
vantageous to  reduce  the  quantity  of  ore  used.  When  silver  alone 
is  being  sought,  the  niter  may  be  entirely  done  away  with  by 
reducing  the  ore  charge  to  a  quantity  sufficient  to  give  a  lead 
button  weighing  not  more  than  30  grams.  In  gold  assays,  how- 
ever, a  charge  less  than  0.5  assay-ton  is  undesirable,  as  it  fails  to 
give  a  sufficiently  close  valuation  of  the  ore. 

The  Soda-Iron  Method. 

The  soda-iron,  or  iron-nail  method  of  assaying  sulphide  ores 
is  radically  different  from  any  of  the  other  methods  so  far 
described.  It  consists  of  a  reducing  fusion  of  the  ore  with  a 
large  amount  of  sodium  carbonate,  as  well  as  a  limited  amount 
of  litharge  and  borax  and  occasionally  a  small  amount  of 
silica,  together  with  an  excess  of  metallic  iron,  usually  in  the 
form  of  nails  or  spikes.  The  principal  difference  between  this 
and  the  other  crucible  methods  consists  in  the  use  of  metallic 
iron  as  a  reducing  and  desulphurizing  agent.  As  iron  reduces 
lead  from  litharge,  as  well  as  from  the  common  lead  minerals, 
*  Jour.  Chem.  Met.  and  Min.  Soc.  of  South  Africa,  2,  p.  465. 


THE  CRUCIBLE  ASSAY  189 

this  latter  reagent  is  limited  to  30  or  35  grams  and  even  less  if 
the  ore  itself  contains  lead.  Therefore,  the  only  basic  fluxes 
available  are  the  alkaline  carbonates,  and  the  quantity  of  these 
to  be  used  is  at  least  two  or  three  times  the  quantity  of  ore.  Just 
before  pouring,  the  excess  of  iron  is  removed. 

Chemical  Reactions.  —  The  chemical  reactions  which  take 
place  in  the  crucible  are  entirely  different  from  those  of  the  other 
crucible  methods.  In  the  case  of  the  niter  and  roasting  methods 
of  assaying,  the  sulphides  of  the  ore  are  oxidized  by  litharge, 
niter,  or  the  oxygen  of  the  air  and  the  sulphur  either  passes  off 
as  S02  or  is  converted  into  SOs,  which  displaces  the  carbonic 
acid  of  the  sodium  carbonate,  forming  sodium  sulphate.  In  the 
iron  assay,  part  of  the  sulphur  in  pyrite  and  some  of  the  other 
sulphides  is  volatilized,  part  of  the  sulphur  is  oxidized  by  the  small 
amount  of  litharge  used  and  the  rest  remains  as  sulphide,  appear- 
ing either  as  an  iron  matte  on  top  of  the  lead  button  or  dissolved 
in  the  excess  of  basic  slag. 

The  following  reactions  will  serve  to  illustrate  the  chemical 
changes  which  take  place: 

FeS2  =  FeS  +  S, 
PbS  +  2PbO  =  3Pb  +  S02, 
Cu2S  +  2PbO  =  2CuPb  +  S02, 
FeS  +  3PbO  =  3Pb  +  FeO  +  S02, 
Fe  +  PbO  =  Pb  +  FeO. 

When  the  litharge  is  all  reduced  the  following  occur: 

PbS  +  Fe  =  Pb  +  FeS  (matte), 

FeS2  +  Fe  =  2FeS  (matte), 

Sb2S3  +  3Fe  =  2Sb  +  3FeS  (matte), 

AsjjSs  +  13Fe  =  2F6&AS  (speiss)  +  3FeS  (matte), 

Cu2S  +  Fe  =  Cu2  +  FeS  (partial). 

Finally,  if  a  sufficient  excess  of  alkaline  flux  is  used,  the  iron 
matte  is  dissolved  by  this  basic  slag,  probably  as  a  double  sul- 
phide of  iron  and  sodium  or  potassium. 

From  the  equations  it  will  be  seen  that  copper,  arsenic  and 
antimony  are  reduced,  at  least  in  part,  and  either  go  into  the  lead 
button,  or  in  the  case  of  arsenic  form  a  speiss  which  appears  as  a 
hard,  white  globule  partly  embedded  in  the  top  surface  of  the  lead 
button. 


190 


A    TEXTBOOK  OF  FIRE  ASSAYING 


In  Table  XXIV  are  shown  the  heats  of  formation  of  some 
of  the  common  metallic  sulphides  expressed  in  terms  of  a  unit 
weight  of  sulphur. 

TABLE  XXIV. 
HEAT  OF  FORMATION  OF  METALLIC  SULPHIDES. 


Formula 

Calories 

Formula 

Calories 

K2S 

103,500 

CoS 

21,900 

CaS 

94,300 

Cu2S 

20,300 

Na2S 

89,300 

PbS 

20,200 

MnS 

45,600 

NiS 

19,500 

ZnS 

43,000 

fSb2S 

11,500 

FeS 

24,000 

Ag2S 

3,000 

A  glance  at  this  table  points  to  the  theoretical  possibility  of 
reducing  the  sulphides  of  cobalt,  copper,  lead,  nickel,  antimony 
and  silver  by  metallic  iron,  and  this  is  borne  out  by  laboratory 
experience.  From  the  thermochemical  data  it  may  also  be 
predicted  that  but  little  zinc  will  be  reduced,  and  therefore  the 
lead  button  will  be  free  from  this  metal. 

Another  reaction  which  is  important  in  this  connection  is  the 
oxidizing  effect  of  alkaline  carbonates  on  metallic  sulphides. 
This  reaction  affords  a  considerable  reduction  of  lead  from  galena 
when  the  latter  is  fused  with  alkaline  carbonate  alone,  and  was  the 
basis  of  the  Upper  Harz  method  for  the  assay  of  lead  in  galena 
ores.  The  reaction  as  given  by  Kerl  is  as  follows : 

7PbS  +  4K2C03  =  4Pb  +  3(K2PbS2)  +  K2SO4  +  4C02. 

At  first  glance  this  reaction  may  not  appear  to  be  reasonable, 
but  a  simple  trial  fusion  with  these  two  substances  will  serve  to 
convince  the  most  skeptical  that  something  very  much  like  this 
does  occur.  Taken  step  by  step,  starting  with  the  reversible 
reaction : 

(1)  3PbS  +  3K2CO3  <=*  3K2S  +  3PbCO3, 

the  explanation  is  simple.      Lead  carbonate  is  readily  dissociated 
by  heat  as  follows : 

(2)  3PbC03  =  3PbO  +  3CO2. 

The  CO2  escapes  and  this  allows  equation  (1)  to  proceed  in  a 
right-handed  direction. 


If0 

~ 

•   a 

THE   CRUCIBLE  ASSAY  191 

The  lead  oxide  resulting  from  equation  (2),  in  the  presence  of 
alkaline  carbonate,  reacts  with  more  lead  sulphide  as  follows: 

(3)  PbS  +  3PbO  -f  K2CO3  =  4Pb  +  K2SO4  +  CO,. 

A  condition  of  equilibrium  appears  to  be  reached  when  the  simple 
double  sulphide  of  alkali  and  lead  is  obtained,  i.e.: 

(4)  3PbS  +  3K2S  =  3(K2PbS2). 

Adding  equations  (1),  (2),  (3)  and  (4)  we  have  KerFs  reaction: 
7PbS  +  4K2CO3  =  4Pb  +  3(K2PbS2)  +  K2SO4  +  4CO2. 

It  is  obvious  that  sodium  carbonate  will  have  the  same  effect 
as  potassium  carbonate. 

Limitations  of  the  Method.  —  The  soda-iron  method  is  an 
excellent  one  for  suitable  ores  when  the  greatest  accuracy  is  not 
desired,  but  is  limited  in  its  application  to  pure  ores.  It  is  known 
to  give  low  silver  results  on  high  sulphide  ores  such  as  nearly  pure 
pyrite,  but  if  properly  conducted  the  results  should  not  be  more 
than  2  or  3  per  cent  lower  than  those  obtained  by  the  niter  method, 
while  the  gold  results  are  but  little  different  in  the  two  cases. 
This  loss  of  silver  is  attributed  by  Hall*  to  the  solubility  of  the 
silver  in  the  iron  sulphide  of  the  slag,  although  according  to 
Fulton f,  "  ferrous  sulphide  has  practically  no  solvent  action  on 
silver  or  on  gold."  The  slag  obtained  in  the  assay  of  pure  pyrite 
contains  a  large  amount  of  ferrous-alkaline-sulphide  and  this 
probably  has  a  slight  solvent  action  on  silver,  so  that  the  silver 
is  distributed  between  the  slag  and  the  lead  button  in  proportion 
to  the  relative  amounts  of  ferrous-alkaline-sulphide  and  lead 
present  and  according  to  its  solubility  in  the  one  as  compared  with 
the  other.  If  this  is  true,  the  less  sulphide  sulphur  the  slag  con- 
tains and  the  greater  the  quantity  of  lead  reduced,  the  higher 
the  silver  recovery  and  the  more  satisfactory  the  results  should 
be.  This  points  'out  one  detail  of  the  furnace  manipulation  of 
the  iron-nail  assay  of  pyritic  ores  which  should  be  carefully  reg- 
ulated, i.e.,  the  temperature  should  be  held  at  a  dull  red  for  some 
time  to  aid  in  the  elimination  of  the  first  atom  of  sulphur  from  the 
pyrite,  which  breaks  up  at  this  temperature.  It  is  also  important 
to  provide  sufficient  litharge  to  supply  a  good-sized  lead  button 
and  more  important  still  to  reduce  this  as  completely  as  possible. 

*  Assay  of  Gold  and  Silver  by  the  Iron-Nail  Method  Trans.  AJ.  M.E., 
47,  p.  37. 

f  Trans.  A.I.M.E.,  39,  p.  596. 


192  A   TEXTBOOK  OF  FIRE  ASSAYING 

A  35-gram  lead  button  is  needed  for  pure  pyrite.  The  excessively 
high  slag  losses  often  reported  for  this  method,  are  probably 
caused  by  too  small  a  lead-fall  and  too  short  a  time  of  fusion, 
which  would  result  in  leaving  some  lead  sulphide,  a  good  solvent 
for  silver,  in  the  slag. 

In  general  it  may  be  said  that  the  method  is  not  suited  for  ores 
carrying  nickel,  copper,  cobalt,  arsenic,  antimony,  bismuth  or 
tellurium.  Even  when  an  ore  contains  several  per  cent  of  copper, 
this  metal  may  not  enter  the  lead  button  in  sufficient  quantity 
to  interfere  seriously  with  cupellation;  but  the  presence  of  copper 
always  gives  low  results,  probably  because  of  the  solvent  action 
of  the  copper  sulphide,  contained  in  the  slag,  upon  the  silver. 
Ores  containing  nickel  are  least  of  all  suited  to  the  method. 

The  Slag.  —  The  slag  made  should  not  be  more  acid  than 
a  mono-silicate,  and  a  sub-silicate  is,  perhaps,  preferable.  The 
slag  does  not  attack  the  crucibles  to  any  extent,  and  the  latter  may 
be  used  a  number  of  times,  if  care  is  taken  to  see  that  they  do  not 
retain  any  lead  shot. 

Atmosphere. —  A  reducing  atmosphere  should  be  maintained 
in  the  furnace  to  prevent  oxidation  and  corrosion  of  the  nails. 
This  may  be  accomplished  by  placing  several  crucibles  contain- 
ing soft  coal  in  the  front  part  of  the  muffle  and  renewing  the  coal 
in  them  if  necessary.  In  an  oxidizing  atmosphere  the  nails  are 
badly  corroded.  The  ferric  oxide  scale  formed  causes  the  slag  to 
become  thick  and  pasty  and  this  tends  to  cause  the  retention  of 
lead  shot  in  the  crucible. 

Procedure.  —  Van  the  ore,  estimate  and  record  its  mineral 
composition.  Note  especially  the  amount  of  lead  minerals.  Use 
a  20-,  25-  or  30-gram  crucible  according  to  the  amount  of  reagents 
required.  The  following  charges  are  suggested  as  capable  of 
yielding  better  results  than  the  customary  30  grams  of  sodium 
carbonate,  10  grams  of  borax-glass,  and  25  grams  of  litharge. 

Half  Galena 

Galena  Half    Pyrite  Pyrite 

Ore  0.5  A.  T.  0.5  A.  T.  0.5  A.  T. 

Sodium  carbonate  30  grams  40  grams  50      grams 

Borax  10         "  15       "  20-25     " 

Litharge  20         "  27       "  35 

Silica  2        "  2      "  2          " 


THE  CRUCIBLE  ASSAY  193 

Insert  from  3  to  5  twenty-penny  cut  nails,  or  preferably  one  3J 
or  4  inch  track  spike,  point  downward. 

Heat  gradually  to  fusion,  fuse  from  forty  to  sixty  minutes. 
Examine  the  nails  occasionally  and  if  they  are  badly  eaten  add 
several  fresh  ones,  leaving  the  old  ones  in  the  crucible  if  they  cannot 
be  removed  free  from  lead.  Fuse  until  the  nails  may  be  freed 
from  lead  by  tapping  them  gently  and  washing  them  around  in 
the  slag.  Remove  all  nails  and  pour  as  usual.  The  slag  will  be 
black  and  should  separate  easily  from  the  lead  button. 

Notes:  1.  If  the  ore  contains  two  or  more  grams  of  silica  none  need  be 
added. 

2.  If  bicarbonate  of  soda  is  substituted  for  the  normal  carbonate  use  a 
correspondingly  greater  weight. 

3.  This  fusion  requires  a  somewhat  longer  time  than  the  niter  fusion, 
owing  to  the  fact  that  time  must  be  allowed  for  all  of  the  charge  to  come  in 
contact  with  the  surface  of  the  iron  nails. 

4.  The  lead  may  not  start  to  drive  in  cupeling  quite  as  rapidly  as  other 
buttons  owing  to  a  small  amount >of  iron  which  is  often  present. 

5.  A  matte  indicates  too  much   silica,  too  little  alkaline  carbonate  or 
too  short  a  time  of  fusion. 

The  Roasting  Method. 

This  method  of  assaying  sulphide  ores  is  rarely  used,  but  may 
be  found  of  advantage  for  very  low-grade  pyritic  ores,  and  will  be 
briefly  described. 

Procedure.  —  Take  from  0.5  to  5.0  assay-tons  of  ore  and 
spread  out  in  a  well-chalked  roasting  dish  of  sufficient  size  to 
allow  of  stirring  without  loss.  Have  the  muffle  at  a  dull  red  only 
and  the  fire  so  low  that  the  temperature  of  the  muffle  may  be 
held  stationary,  or  raised  slowly.  Place  the  dish  in  the  muffle  and, 
if  the  ore  contains  minerals  which  decrepitate,  cover  it  and  keep 
it  covered  until  danger  from  this  source  is  passed.  The  ore  should 
soon  begin  to  roast.  When  fumes  are  noticed  coming  from  the 
ore,  check  the  fire  and  hold  it  at  this  temperature  for  some  time, 
stirring  frequently.  After  all  danger  of  fusing  is  over,  gradually 
raise  the  temperature,  stirring  at  intervals  of  twenty  minutes 
or  half  an  hour.  Finally  heat  to  about  700°  C.  for  half  an  hour. 
If  the  ore  contains  only  sulphides  of  iron  and  copper,  practically 
all  of  the  sulphur  will  be  removed  withfn  this  time.  If  there  is 
any  doubt  about  the  roast  being  complete,  remove  from  the 
muffle,  add  a  small  amount  of  charcoal  and  see  if  there  is  any  odor 
of  sulphur  dioxide.  If  the  ore  contains  zinc,  a  much  higher 


194  A    TEXTBOOK  OF  FIRE  ASSAYING 

temperature  will  be  required  to  break  up  the  zinc  sulphate.  It  is 
not  advisable,  however,  to  carry  the  roasting  temperature  above 
700°  C.  For  ores  which  consist  principally  of  pyrite,  galena  or 
stibnite,  place  a  weighed  amount  of  silica  on  the  dish  before  intro- 
ducing the  ore.  A  weight  of  silica  equal  to  that  of  sulphide  may 
be  used.  This  will  serve  to  prevent  the  roasted  material  from 
adhering  to  the  dish  and  will  be  found  useful  as  a  flux  in  the 
subsequent  fusion. 

If  the  ore  contains  arsenic  or  antimony,  the  roasting  operation 
is  more  difficult.  The  best  conditions  for  the  elimination  of  these 
elements  are  alternate  oxidation  and  reduction  at  a  low  tempera- 
ture. The  presence  of  sulphur  aids  in  the  elimination  of  these 
elements,  because  their  sulphides  are  volatile.  To  obtain  the 
reducing  action  necessary  for  the  elimination  of  arsenic  and 
antimony,  take  the  partially  roasted  ore  from  the  muffle,  allow 
it  to  cool  for  a  few  moments,  and  then  mix  powdered  charcoal 
or  coal  dust  with  it  and  roast  at  a  dull  red  heat  until  the  coal  is 
burned  off.  Then  add  more  coal  and  reroast.  Repeat  this  until 
no  more  fumes  of  arsenic  or  antimony  are  noticed,  then  heat  with 
frequent  stirring  to  about  700°  C. 

After  the  ore  is  roasted,  the  dish  is  carefully  cleaned  out  and  the 
ore  is  charged  into  a  crucible  with  fluxes  and  treated  exactly  as  a 
Class  1  ore.  If  the  sulphide  mineral  was  mostly  iron,  the  ore  will 
probably  be  found  to  have  a  slight  oxidizing  power  due  to  the 
formation  of  Fe2O3  and  Fe3O4  in  the  roasting. 

The  roasting  method  of  assaying  is  slow  and  takes  up  much 
muffle  space.  It  is  open  to  the  liability  of  serious  mechanical 
and  volatilization  losses.  Its  most  useful  field  would  seem  to  be 
the  assay  of  low-grade  pyritic  gold  ores  where  a  very  accurate 
determination  of  gold  is  desired.  The  method  usually  gives  low 
results  in  silver. 

The  Combination  Wet-and-Fire  Assay. 

The  combination  wet-and-fire  assay  is  used  principally  for 
the  determination  of  gold  and  silver  in  impure  ores,  matte, 
speiss  and  bullion.  A  description  of  the  method,  as  applied  to 
the  assay  of  ores  containing  cobalt,  nickel  and  arsenic,  will  be 
found  in  the  chapter  on  "The  Assay  of  Complex  Ores,"  and  the 
application  of  the  method  to  the  assay  of  copper  bullion  may  be 
found  in  the  chapter  on  "The  Assay  of  Bullion." 


THE  CRUCIBLE  ASSAY  195 

ASSAY  OF   CLASS  3   ORES. 

The  principal  ores  belonging  to  this  class  are  those  containing 
some  of  the  higher  oxides  of  iron  or  manganese,  i.e.,  Fe2O3,  Fe3O4, 
Mn02.  These  are  reduced  by  carbon  and  tend  to  enter  the  slag 
as  ferrous  and  manganous  silicates  respectively.  If  the  charge 
made  up  for  these  ores  contained  only  the  ordinary  amount  of 
flour,  all  of  this  might  be  used  up  in  reducing  the  oxides  of  the 
ore  and  no  lead  button  would  result.  To  remedy  this,  the  oxidizing 
power  of  the  ore  should  be  known  before  the  charge  is  made  up. 

To  determine  the  oxidizing  power  of  an  ore,  fuse  a  known  weight 
of  it,  say  10  or  20  grams,  with  a  regular  crucible  charge  for  that 
amount  of  ore  and  a  carefully  weighed  amount  of  argols  or  flour 
of  known  reducing  power,  more  than  sufficient  to  oxidize  the  ore. 
The  weight  of  lead  that  the  argols  may  be  supposed  to  have  re- 
duced from  an  excess  of  litharge,  minus  the  weight  of  lead  ob- 
tained, is  evidently  the  amount  oxidized  by  the  ore.  This  weight 
divided  by  the  weight  of  ore  taken  gives  the  oxidizing  power. 

When  the  oxidizing  power  of  the  ore  has  been  determined  the 
assay  is  made  in  the  same  manner  as  for  Class  1  ores,  with  the 
addition  of  the  extra  flour  required. 


CHAPTER  IX. 

THE  ASSAY  OF  COMPLEX  ORES  AND  SPECIAL 
METHODS. 

THE  ASSAY   OF   ORES   CONTAINING  NICKEL  AND   COBALT. 

Ores  from  the  Cobalt  district  of  Ontario  present  unusual  diffi- 
culties for  the  assayer,  as  well  as  for  the  metallurgist.  The 
high-grade  ore,  which  carries  several  thousand  ounces  of  silver 
per  ton,  is  an  intimate  mixture  of  the  arsenides  and  sulphides  of 
cobalt,  nickel  and  silver  with  a  large  amount  of  what  appears  to 
be  native  silver,  but  actually  consists  of  an  alloy  of  silver  with 
arsenic,  nickel  and  cobalt. 

The  question  of  determining  the  amount  of  silver  in  a  shipment 
of  such  ore  is  actually  more  of  a  sampling  than  an  assaying  prob- 
lem. The  accepted  method  of  sampling  consists  in  crushing  the 
entire  lot  of  ore  to  a  relatively  small  size  and  separating  the  me- 
tallic from  the  non-metallic  portions.  Each  portion  is  then  as- 
sayed separately  and  the  results  combined  to  give  the  average 
silver  content  of  the  ore.  For  a  more  detailed  account  of  the 
sampling  of  such  an  ore  the  student  is  referred  to  Volume  11, 
pages  287  to  293  inclusive,  of  the  Journal  of  the  Canadian  Mining 
Institute  where  the  practice  at  the  Copper  Cliff  smelter  is  de- 
scribed. A  later  paper  describing  the  method  used  at  the  Cobalt 
sampler  may  be  found  in  the  Transactions  of  the  Canadian 
Mining  Institute,  Volume  17,  pages  199  to  251  inclusive. 

For  low-grade  ores  containing  but  little  nickel,  the  crucible 
method  of  assaying  will  give  satisfactory  results.  For  details 
reference  may  be  made  to  an  article  on  this  subject  in  the  En- 
gineering and  Mining  Journal,  Volume  90,  page  809. 

For  high-grade  ores,  a  properly  conducted  combination  method 
will  yield  higher  and  more  concordant  results  than  can  be  ob- 
tained by  any  "  all-fire  "  method.  The  following  method  of 
A.  M.  Smoot  is  taken  from  his  discussion*  of  this  problem. 

*  Trans.  Can.  Min.  Inst.  17,  pp.  244-250. 
196 


ASSAY  OF  COMPLEX  ORES  AND  SPECIAL  METHODS    197 

The  Combination  Assay.  —  Quarter-  or  half-  assay-ton  portions 
of  the  pulp  are  taken,  the  former  weight  if  the  sample  contains 
over  2000  ounces  per  ton,  the  latter  if  the  silver  is  less  than  this. 
The  pulp  is  treated  in  beakers  with  strong  nitric  acid,  added  a 
little  at  a  time  until  danger  of  frothing  is  past.  About  75  c.c. 
of  acid  is  required  for  0.25  A.  T.  portions  and  100  c.c.  for  0.5 
A.  T.  portions.  The  solutions  are  heated  on  a  steam  bath  until 
red  fumes  cease  to  be  generated  and  are  then  diluted  with  200 
c.c.  of  distilled  water  and  allowed  to  stand  until  cold,  preferably 
over  night.  It  is  very  important  that  the  solutions  be  allowed 
to  stand  before  they  are  filtered,  because  with  certain  ores  con- 
taining much  arsenic  together  with  some  antimony  and  lime,  a 
white  crystalline  coating  appears  on  the  bottoms  and  sides  of  the 
beakers  and  cannot  be  detached  by  washing  or  even  scraping. 
This  coating  contains  a  little  silver,  and  if  it  is  not  allowed  to 
form  in  the  original  nitric  acid  solution  it  forms  later  on  in  the 
process  and  makes  trouble.  Insoluble  residues  are  filtered  off 
and  washed  thoroughly.  If  there  is  any  coating  on  the  sides  and 
bottoms  of  the  beakers  which  cannot  be  readily  detached  with  a 
piece  of  filter  paper,  it  is  treated  in  the  beaker  with  a  hot  solution 
of  caustic  soda  which  quickly  disintegrates  it.  The  caustic  soda 
solution  is  acidulated  with  a  little  nitric  acid  and  washed  into 
the  filter  with  the  insoluble  residue.  Most  of  the  silver  is  dissolved 
by  the  original  nitric  acid  treatment  and  passes  through  the  filters 
as  silver  nitrate,  but  a  little  remains  with  the  insoluble  residue. 
If  the  insoluble  residues  are  large  in  amount  they  are  dried  and 
burned  in  crucibles,  fused  with  sodium  carbonate,  borax-glass, 
litharge  and  a  reducing  agent.  If  they  are  small  they  are  dried 
and  burned  in  scorifiers  and  scorified  with  test  lead  and  borax- 
glass.  In  either  case,  the  lead  buttons  from  the  insolubles  are 
reserved.  Standard  sodium  chloride  solution  is  added  to  the 
nitric  acid  solutions  in  amount  sufficient  to  precipitate  all  of  the 
silver  as  chloride,  but  any  considerable  excess  of  the  precipitant 
is  to  be  avoided.  The  silver  chloride  is  stirred  briskly  until  it 
agglomerates  and  is  then  allowed  to  stand  for  an  hour  until  it 
settles  and  the  supernatant  liquid  becomes  clear.  If  it  remains 
cloudy,  rapid  stirring  is  repeated  and  it  is  again  allowed  to  settle. 
The  clear  solutions  are  filtered  through  double  filter  papers  and 
the  silver  chloride  precipitates  transferred  to  the  filters  by  a 
water  jet  and  there  washed  slightly  with  water.  The  beakers  are 


198  A   TEXTBOOK  OF  FIRE  ASSAYING 

washed  well  with  a  wash-bottle  jet  and  any  traces  of  silver  chlor- 
ide remaining  in  them  are  wiped  off  with  small  pieces  of  filter 
paper  which  are  placed  in  the  filters.  Filters  containing  the  silver 
chloride  are  transferred  to  scorifiers  which  have  been  glazed  on 
the  inside  by  melting  litharge  in  them  and  pouring  away  the  excess. 
The  glazing  is  done  to  prevent  the  porous  scorifiers  from  absorb- 
ing moisture  from  the  damp  paper,  and  as  a  further  protection, 
a  small  disc  of  pure  sheet  lead  is  placed  beneath  the  filter  papers. 
The  scorifiers  are  transferred  to  a  closed  oven  heated  to  about 
250°  -  300°  C.,  where  they  are  dried  and  the  paper  is  slowly 
charred  until  it  is  practically  all  consumed.  This  method  of 
burning  the  filter  papers  is  an  essential  step,  since  it  avoids  losses 
of  silver  chloride  which  are  apt  to  occur  if  the  burning  is  done 
rapidly  in  a  muffle.  Fine  test  lead  is  sprinkled  over  the  burned 
silver  chloride  residues  and  the  lead  buttons  resulting  from  the 
crucible  fusions  or  scorifications  of  the  corresponding  insoluble 
residues  are  added.  Scorification  is  then  conducted  at  a  low 
temperature  so  as  to  obtain  15-gram  lead  buttons.  These  are 
cupeled  at  a  low  temperature,  care  being  taken,  in  the  case  of 
large  silver  beads,  to  avoid  "  spitting  "  at  the  end  of  cupellation. 

The  combination  method  is  acceptable  to  the  smelters  since  it 
does  not  include  slag  and  cupel  corrections.  Inasmuch  as  all 
impurities  likely  to  effect  variations  in  the  volatilization  and 
slag  losses  are  removed  prior  to.  the  fire  work,  the  results  of  assays 
made  on  different  days  and  in  different  muffles,  under  different 
conditions,  are  more  uniform  than  when  the  untreated  ores  are 
assayed  directly. 

Small  amounts  of  bismuth  occurring  in  the  Cobalt  silver  ores 
are  a  source  of  irregularity  in  "  all-fire  "  methods  because  bismuth 
is  retained  to  some  extent  by  silver  after  cupellation.  In  the 
combination  method,  bismuth  is  eliminated  before  any  fire  work 
is  done. 

THE  ASSAY  OF  TELLURIDE  ORES. 

The  determination  of  the  precious  metals  in  ores  containing 
tellurium  has  always  been  considered  more  than  ordinarily  difficult. 
Results  obtained  by  different  assayers  and  even  duplicate  assays 
by  the  same  man  have  often  been  widely  divergent.  The  litera- 
ture of  telluride  ore  assaying  is  extensive  and  none  too  satisfactory; 
however,  it  is 'safe  to  say  that  most  of  the  reported  differences 


ASSAY  OF  COMPLEX  ORES  AND  SPECIAL  METHODS    199 

between  duplicates  and  between  different  assayers  have  been  due 
more  to  difficulties  in  sampling  than  to  the  chemical  interference 
of  the  element  tellurium.  When  it  is  considered  that  most  of 
the  telluride  ores  which  are  mined  contain  less  than  0.1  per  cent 
of  telluride  mineral,  it  is  apparent  that  more  than  ordinary  care 
must  be  taken  to  ensure  obtaining  a  fair  proportion  of  this  in  the 
final  assay  portion.  The  telluride  mineral  itself  may  contain  as 
much  as  40  per  cent  of  gold,  so  that  one  100-mesh  particle  more 
or  less  in  the  assay  portion  may  make  a  difference  of  several 
hundredths  ounces  of  gold  to  the  ton.  To  obviate,  as  far  as 
possible,  this  lack  of  homogeneity,  all  telluride  ores  should  be 
pulverized  to  at  least  150-  and  preferably  200-mesh  and  then 
very  thoroughly  mixed  before  the  assay  portions  are  weighed  out. 

Effect  of  Tellurium.  —  Tellurium  is  a  close  associate  of  both 
gold  and  silver  and  is  difficult  to  separate,  from  these  metals 
either  in  the  crucible,  scorification  or  cupellation  processes.  It 
is  not,  however,  often  found  in  abundance,  and  even  in  high- 
grade  ores  tellurium  itself  is  found  in  comparatively  small  amounts. 
For  instance,  in  two  high-grade  ores  used  by  Hillebrand  and 
Allen*  in  their  experiments  on  the  assay  of  telluride  ores,  con- 
taining respectively  15  and  19  ounces  of  gold  per  ton,  there  was 
tellurium  amounting  to  0.074  and  0.092  per  cent  respectively. 
It  seems  unreasonable  to  expect  such  small  quantities  of  any  ele- 
ment to  influence  seriously  the  results  of  a  fire-assay. 

In  order  to  study  the  effects  of  tellurium  in  the  gold  and  silver 
assay  it  is  necessary  to  experiment  with  ores  or  alloys  containing 
much  more  tellurium  than  those  above  mentioned.  The  fol- 
lowing facts  regarding  the  behavior  of  tellurium  in  cupellation 
and  fusion  are  mostly  due  to  the  work  of  Holloway,f  Peasef  and 
Smith, {  whom  we  have  to  thank  for  coordinating  and  elucidating 
much  information  which  was  hitherto  much  scattered  and  of 
doubtful  value. 

Effect  of  Tellurium  in  Cupellation.  —  The  presence  of  tellurium 
in  a  lead  button  causes  a  weakening  of  the  surface  tension  of  the 
molten  metal.  The  result  is  that  the  metal  tends  to  "  wet  "  the 

*  Bull.  253,  U.  S.  Geol.  Survey. 

t  The  assay  of  Telluride  Ores,  G.  T.  Holloway  and  L.  E.  B.  Pease,  Trans. 
I.  M.  M.,  17,  p.  175. 

|  The  Behavior  of  Tellurium  in  Assaying,  Sydney  W.  Smith,  Trans. 
I.  M.  M.,  17,  p.  463. 


200  A   TEXTBOOK  OF  FIRE  ASSAYING 

surface  of  the  cupel,  and  this  allows  some  particles  of  alloy  to 
pass  into  the  cupel  while  others  are  left  behind  to  cupel  by  them- 
selves on  its  surface  and  form  minute  beads.  In  the  case  of  a 
button  containing  10  per  cent  or  more  of  tellurium  with  an  equal 
weight  of  gold  or  silver,  complete  absorption  may  take  place. 
As  the  proportion  of  lead  in  the  alloy  is  increased,  the  amount  of 
absorption  becomes  less;  when  the  lead  amounts  to  eighty  times 
the  tellurium  very  little  loss  of  precious  metal  occurs  in  a  properly 
conducted  cupellation. 

Tellurium  is  removed  comparatively  slowly  during  cupellation, 
particularly  in  the  early  stages,  as  might  be  expected  on  comparing 
the  heat  of  formation  of  its  oxide  with  that  of  lead  oxide.  Rose* 
gives  the  following  figures  for  the  heat  of  combination  of  these 
metals  with  16  grams  of  oxygen,  — Pb  to  PbO  5030  calories, 
Te  to  Te(>2  3860  calories.  To  avoid  danger  of  undue  loss  in 
cupellation  of  buttons  from  the  assay  of  such  ores,  as  much  as 
possible  of  the  tellurium  should  be  removed  before  cupellation. 
It  is  also  evident  that  the  assayer  should  allow  for  large  lead 
buttons  in  order  that  the  ratio  of  lead  to  tellurium  may  be  high. 

Silver  in  the  alloy  protects  gold  from  losses  due  to  the  presence 
of  tellurium.  It  appears  to  act  as  a  diluent  for  the  gold  and 
should  always  be  added  to  every  gold  assay  for  this  reason,  if  for 
no  other. 

In  the  case  of  imperfect  cupellation,  tellurium  is  retained  by 
the  bead  and  gives  it  a  frosted  appearance.  In  perfect  cupel- 
lation the  final  condition  of  the  tellurium  is  that  of  complete 
oxidation  to  TeO2.  Owing  to  its  effect  in  reducing  surface  ten- 
sion, as  a  result  of  which  minute  beads  are  often  left  behind,  it 
would  be  well  to  use  a  cupel  having  a  finer  surface  when  cupeling 
buttons  containing  tellurium.  Smith  states  that  the  loss  due  to 
subdivision  and  absorption  in  this  case  is  much  less  when  a  "  pat- 
ent "  (magnesia)  cupel  is  used.  Losses  of  gold  and  silver  by 
volatilization,  during  properly  conducted  cupellation  of  lead 
buttons  from  ordinary  telluride  ores,  is  extremely  small. 

Effect  of  Tellurium  in  Fusions.  —  Tellurium  was  formerly 
believed  to  be  oxidized  to  the  dioxide  during  fusion  and  to  go 
into  the  slag  as  a  sodium  or  lead  tellurate.  Smith  disagrees  with 
this  and  argues  that  tellurates  are  decomposed  at  a  red  heat,  and 
that  lead  tellurate  is  white,  while  he  found  the  litharge  slags  ob- 
*  Trans.  Inst.  Min.  Met.,  14,  p.  384. 


ASSAY  OF   COMPLEX  ORES  AND  SPECIAL  METHODS   201 

tained  in  the  fusion  of  telluride  compounds  to  be  black.  He 
believes  that  tellurium  exists  in  the  slag  as  the  black  monoxide, 
TeO. 

The  slag  best  suited  to  the  oxidation  and  retention  of  tellurium 
in  crucible  assaying  is  a  basic  one  containing  a  considerable  excess 
of  litharge.  The  temperature  of  fusion  should  be  moderately  low, 
as  a  high  temperature  prevents  the  satisfactory  oxidation  and 
slagging  of  the  tellurium,  owing  probably  to  the  formation  of 
lead  silicates  before  the  litharge  has  had  time  to  oxidize  the 
tellurium.  Smith  gives  the  following  reaction  for  the  oxidation 
of  tellurium : 

2PbO  +  Te  =  Pb2O  +  TeO. 

In  support  of  this  he  claims  to  have  found  the  black,  suboxide 
of  lead  in  the  slag. 

Practically  all  authorities  agree  that  the  scorification  process  is 
not  reliable  for  telluride  ores.  When  a  button  from  a  crucible 
assay  contains  too  much  tellurium  for  direct  cupellation  Smith 
recommends  fusing  or  "  soaking  "  the  button  under  an  ample 
amount  of  litharge  at  a  moderate  temperature  i.e.,  700-900°  C. 

Hillebrand  arid  Allen  used  the  following  charge  for  ores  con- 
taining from  15  to  19  ounces  of  gold  and  0.074  to  0.092  per  cent  of 
tellurium. 

Ore 1.  A.  T. 

Sodium  carbonate       30  grams 

Borax-glass 10      " 

Litharge 180      " 

Reducing  agent,  .for  25-gram  buttons 
Silver 2|  to  3  times  gold 

They  find  slag  losses  no  higher  than  with  ordinary  gold  ores 
and  no  serious  cupellation  losses.  With  ores  containing  much 
more  tellurium  than  the  above,  the  quantity  taken  should  be 
reduced  and  the  rest  Of  the  charge  maintained  as  before. 

THE  ASSAY  OF  ORES  AND   PRODUCTS  HIGH  IN 
COPPER. 

Crucible  methods  for  the  assay  of  matte  and  ores  high  in  copper 
have  largely  supplanted  the  older  scorification  method.  This 
is  due  to  the  fact  that  a  larger  amount  of  pulp  may  be  used  for 


202  A    TEXTBOOK  OF  FIRE  ASSAYING 

each  individual  assay,  thus  increasing  the  accuracy  of  the  results. 
The  copper  is  eliminated,  as  it  is  in  the  scorification  assay,  by  the 
solution  of  its  oxide  in  the  basic  lead  oxide  slag.  The  assay  thus 
combines  the  advantages  of  the  scorification  with  those  of  the 
crucible  assay. 

Perkins*  has  made  a  careful  study  of  this  process,  and  calls 
attention  to  the  fact  that  the  litharge  used  must  be  in  proportion 
to  the  amount  of  copper  and  other  impurities  in  the  ore.  The 
amounts  he  uses  are  very  large,  from  137  to  300  parts  PbO  to  1 
part  Cu,  and  make  the  method  an  expensive  one.  Others  have 
reduced  this  amount  considerably,  and  still  manage  to  get  buttons 
which  will  cupel. 

The  Slag.  —  The  slag  should  be  decidedly  basic,  for  if  the 
litharge  is  combined  with  large  amounts  of  silica  and  borax,  it 
will  no  longer  retain  its  power  of  holding  the  copper  in  solution. 
A  small  amount  of  silica  is  necessary  to  prevent,  to  some  extent, 
the  action  of  the  litharge  upon  the  crucible.  One  part  of  silica 
to  from  15  to  20  parts  of  litharge  is  generally  allowed  in  the  charge. 
Borax  should  be  entirely  omitted  as  it  decreases  the  copper-hold- 
ing capacity  of  the  slag,  and  also  causes  boiling  of  the  charge. 
Perkins  states  that  the  best  results  are  obtained  with  a  slag  which 
exhibits,  when  cooled  and  broken,  a  somewhat  glassy  exterior 
gradually  passing  to  litharge-like  crystals  towards  the  center. 
The  amount  of  crystallization  which  takes  place  is,  of  course, 
a  function  of  the  rate  of  cooling  and  will  depend  among  other 
things  upon  the  size  of  the  charge,  the  temperature  of  the  charge 
when  poured,  and  of  the  mold,  so  that  too  much  weight  should 
not  be  given  to  the  above.  The  slag  should,  however,  be  crystal- 
line and  resemble  litharge;  a  slag  which  is  dull  or  glassy  throughout 
indicates  the  presence  of  too  much  acid  for  a  good  elimination  of 
copper. 

Conduct  of  the  Assay.  —  On  account  of  the  very  corrosive  action 
of  the  litharge  slag  it  is  especially  necessary  that  the  fusion  be 
made  rapidly.  The  muffle  should  be  hot  to  start,  1000°  to  1 100°  C., 
the  hotter  the  better,  and  the  fusion  should  be  finished  in 
from  twenty  to  thirty  minutes.  This  not  only  preserves  the 
crucibles,  but  also,  as  a  necessary  sequel,  prevents  the  slag  from 

*  The  Litharge  Method  of  Assaying  Copper-Bearing  Ores  and  Products, 
and  the  Method  of  Calculating  Charges,  W.  G.  Perkins,  Trans.  A.I.M.E.,  31, 
p.  913. 


ASSAY  OF  COMPLEX  ORES  AND  SPECIAL  METHODS   203 


becoming  charged  with  silica  and  thus  forcing  the  copper  into 
the  button.  The  slag  melts  at  a  low  temperature  and  a  very 
high  finishing  temperature  is  not  necessary.  With  a  quick  fusior 
there  is  less  chance  for  oxidation  of  lead  with  the  consequent 
reduction  of  too  small  a  lead  button. 

For  the  best  work  the  hole  in  the  back  of  the  muffle  should  be 
closed  and  a  reducing  atmosphere  maintained  in  the  muffle. 
This  may  be  accomplished  by  filling  the  mouth  of  the  muffle  with 
charcoal  or  coke,  or  by  placing  a  few  crucibles  partly  full  of  soft 
coal  near  the  front  of  the  muffle  and  using  a  tight-fitting  door. 
If  this  precaution  is  not  observed  part  of  the  silver  will  be  oxi- 
dized and  lost  in  the  slag. 

The  following  charges  kindly  furnished  by  the  Boston  and 
Montana  Reduction  Department  of  the  Anaconda  Copper  Mining 
Company,  Great  Falls,  Montana,  are  recommended  for  these  ores. 

TABLE  XXV. 
CHARGES  FOR  COPPER-BEARING  MATERIAL. 


Approximate 
Material                       analysis 

Charge  for  silver 
(In  20-gram  crucible) 

Charge  for  gold 
(In  30-gram  crucible) 

Cu         9-15  per  cent 
SiOt     15-23 
Concen-  FeO     33-40 
trates    S          33^0 
Ag          3-5  ounces 
Au    0.015-0.025  ounces 

Sample                    1  A.  T. 
Soda                      20  grams 
Litharge              100 
Silica                      5 
Niter                  15-25       " 
Cover  mixture 

Sample                    1  A.  T. 
Soda                       30  grams 
Litharge               150 
Silica                      8 
Niter               40-60 
Cover  mixture 

Cu        30-45  per  cent 
Fe        40-30      " 
Matte        S           30-27      " 
Ag        10-18  ounces 
Au     0.07-0.  11  ounces 

Sample                    i  A.  T. 
Soda                      18  grams 
Litharge               100        " 
Silica                      7 
Niter                       6 
Cover  mixture 

Sample                    \  A.  T. 
Soda                      25  grams 
Litharge              200 
Silica                     12 
Niter                     18 
Cover  mixture 

Cu        45-60  per  cent 
Fe        30-15 
Matte        S          27-24 
Ag        15-25  ounces 
Au     O.iO-0.14  ounces 

Sample                    i  A.  T. 
Soda                       18  grams 
Litharge              "25 
Silica                      7 
Niter                      4 
Cover  mixture 

Sample                   \  A.  T. 
Soda                       25  grams 
Litharge              240 
Silica                     12 
Niter                    14 
Cover  mixture 

The  cover  consists  of  one-quarter  inch  of  a  mixture  of  4  parts 
sodium  carbonate,  2  parts  borax  and  1  part  silica.  Fusions  in 
20-gram  crucibles  require  about  thirty  minutes,  those  in  30-gram 


204  A   TEXTBOOK  OF  FIRE  ASSAYING 

crucibles  about  fifty  minutes.  It  will  be  noticed  that  occasion- 
ally as  much  as  60  grams  of  niter  is  used  in  a  single  fusion.  With 
the  proper  muffle  temperature  there  is  said  to^be  no  danger  of  a 
crucible  boiling  over  even  though  the  crucible  be  filled  to  within 
half  an  inch  of  the  top. 

ASSAY   OF  ZINC-BOX  PRECIPITATE. 

The  gold  and  silver  precipitated  from  cyanide  solutions  by 
means  of  zinc  always  contains  more  or  less  metallic  zinc  as  well 
as  more  or  less  copper,  lead  and  other  readily  reducible  metals 
which  may  be  present  in  the  ore,  or  which  may  have  been  intro- 
duced during  the  process.  Gold  precipitate  usually  contains  a 
good  deal  of  metallic  zinc  and  is  generally  given  a  preliminary  acid 
treatment  before  being  melted.  Silver  precipitate,  on  the  other 
hand,  is  comparatively  free  from  zinc  and  may  be  melted  directly. 
Besides  metals,  the  precipitate  may  also  contain  hydroxide, 
cyanide  and  ferro-cyanides  of  zinc,  as  well  as  iron  oxide,  silica, 
alumina,  etc. 

The  materials  as  received  by  the  assayer  will  usually  have  been 
passed  through  a  16-  or  20-mesh  screen  for  the  purpose  of  re- 
moving the  short  zinc,  and  may  or  may  not  have  been  acid-treated. 
The  peculiarities  of  this  material  are  (a)  the  presence  of  more 
or  less  metallic  zinc  which  has  a  reducing  power  of  3.17  and  which 
boils  at  930°  C.,  (6)  the  presence  of  various  compounds  containing 
zinc  oxide,  which  is  difficultly  soluble  in  litharge,  (c)  its  richness 
and  spotty  character,  which  necessitate  the  most  painstaking 
care  to  secure  commercially  satisfactory  results. 

On  account  of  the  amount  of  gold  and  silver  contained,  the 
sampling  and  grinding  should  be  carried  out  in  a  special  room, 
well  separated  from  the  regular  assay  office,  to  avoid  danger  of 
salting.  A  corner  of  the  clean-up  and  melting  room  may  be 
used  if  available,  and  there  should  be  provided  for  this  purpose 
a  special  bucking  board,  as  well  as  special  samplers,  screens, 
brushes,  etc. 

The  assay  sample,  weighing  2  or  3  pounds,  should  be  thoroughly 
dried  and  ground  to  pass  at  least  80-mesh.  A  convenient  quan- 
tity of  the  final  pulp  is  150  or  200  grams,  and  the  80-mesh  sample 
may  be  cut  down  to  this  and  then  ground,  preferably  on  the 
bucking  board,  to  at  least  150-mesh.  This  final  sample  should 
be  thoroughly  mixed  and  dried  again,  cooled  in  a  desiccator  and 


ASSAY   OF  COMPLEX  ORES  AND  SPECIAL   METHODS   205 

kept  there  until  the  final  samples  are  weighed.  This  precaution 
is  observed  both  to  prevent  the  material  from  taking  on  mois- 
ture from  the  air  and  to  prevent  oxidation  of  the  zinc,  which  in 
some  cases  would  cause  a  measurable  error  due  to  change  in  weight 
of  the  sample. 

The  fine  pulp  may  be  assayed  by  crucible  fusion  and  cupellation 
or  by  one  of  several  wet  or  wet-and-fire  methods.  The  crucible 
assay  is  always  corrected  by  a  reassay  of  the  slag  and  corrections 
are  also  applied  for  cupel  absorption. 

The  following  crucible  charge  is  recommended  by  Magenan,* 

Precipitate 0.1  assay-ton 

Sodium  carbonate .         5  grams 

Borax-glass 2       " 

Litharge 70       " 

Flour for  25-30-gram  buttons 

Silica 5  grams 

A  thoroughly  glazed  crucible  should  be  used  for  this  purpose, 
to  ensure  against  any  of  the  precipitate  adhering  to  the  walls 
above  the  level  of  the  fusion.  A  narrow-bladed  spatula  is  con- 
venient for  sampling  the  precipitate.  The  weighing  should  be 
done  on  an  analytical  or  exceptionally  accurate  pulp  balance. 
It  is  customary  to  make  at  least  six  assays  and  to  average  the 
results.  The  fusions  should  be  heated  rather  gradually  to  the 
full  temperature  of  the  muffle. 

According  to  Layng,f  a  high  temperature  at  the  beginning  is 
productive  of  low  results.  Apparently  it  is  better  to  oxidize 
the  metallic  zinc  with  litharge  than  to  allow  it  to  volatilize. 

For  silver-bearing  precipitate  the  Volhard  or  Gay-Lussac 
volumetric  methods  may  be  used,  but  the  latter  should  be  avoided 
in  the  presence  of  mercury,  which  interferes.  There  is  no  great 
advantage  in  the  combination  wet-and-fire  methods  unless  the 
precipitate  contains  considerable  copper  or  other  metals  which 
might  contaminate  the  bead  or  cause  extra  losses  in  cupellation. 

ASSAY   OF  ANTIMONIAL   GOLD   ORES. 

The  niter  method  is  universally  recognized  as  being  the  best 
method  for  the  sulphide  ores  of  antimony.  Considerable  litharge 

*  Min.  and  Sci.  Press,  80,  p.  464. 

f  Mexican  Mining  Journal,  Feb.  1913,  p.  90. 


206  A   TEXTBOOK  OF  FIRE  ASSAYING 

is  necessary  to  keep  the  antimony  out  of  the  lead  button.     The 
following  charge  is  recommended  by  two  English  authorities:* 

Ore 0.5  A.  T. 

Na-jCOs 10-20  grams 

Borax-glass.        5-10       " 

Litharge 100-120  grams 

Niter 19       " 

Silica 10       " 

A  preliminary  assay  to  determine  the  reducing  power  is  of 
course  necessary.  The  above  charge  will  be  found  to  correspond 
almost  exactly  with  our  standard  for  sulphide  ores,  with  litharge 
according  to  Lodge's  rule. 

George  T.  Holloway,  in  discussing  this  method,  recommended 
the  use  of  a  much  larger  proportion  of  soda  in  the  charge,  i.e., 
three  times  as  much  as  stibnite,  in  order  to  aid  in  the  retention  of 
the  antimony  in  the  slag  as  a  sodium  antimonate. 

ASSAY   OF  AURIFEROUS  TINSTONE. 

C.  O.  Bannister f  finds  a  crucible  assay  with  the  following  charge 
to  be  the  most  satisfactory  method : 

Ore 25      grams 

Sodium  carbonate  .  .40 

Borax 10          " 

Red  lead. 60 

Charcoal 1.5       " 

In  this  method  the  tin  is  converted  into  a  fusible  sodium  stan- 
nate.  The  author  found  no  tin  reduced  during  the  fusion,  as 
shown  by  the  fact  that  the  button  cupeled  without  difficulty.  In 
all  ores  carrying  over  1  ounce  of  gold  per  ton,  the  slags  were  cleaned 
by  a  second  fusion  with  10  grams  of  soda,  30  grams  of  red  lead  and 
1.5  grams  of  charcoal. 

Various  other  methods  of  assay  were  tested  but  none  were  as 
satisfactory  as  this. 

CORRECTED   ASSAYS. 

In  the  assay  of  high-grade  ores  and  bullion  it  is  often  desirable 
to  make  a  correction  for  the  inevitable  slag  and  cupel  losses. 

*  William  Kitto,  Trans.  Inst.   Min.  Met.,  16,  p.  89. 
William  Smith,  Trans.  Inst.  Min.  Met.,  9,  p.  332. 
t  Trans.  Inst.  Min.  Met.  (London)  16,  p.  513. 


ASSAY  OF  COMPLEX  ORES  AND  SPECIAL  METHODS    207 

This  is  done  in  one  of  two  ways:  either  by  the  use  of  a  "  check  " 
or  synthetic  assay,  or  by  assaying  the  slags  and  cupels  resulting 
from  the  original  or  commercial  assays. 

In  correcting  by  a  "  check  "  assay,  a  preliminary  assay  is  first 
made  and  then  an  amount  of  proof  silver  or  gold,  or  both,  approxi- 
mately equivalent  to  the  amount  present  in  the  sample,  is  weighed 
out  and  made  up  to  approximately  the  composition  of  the  sample 
by  the  addition  of  base  metal,  etc.  The  check  thus  made  is 
assayed  in  the  same  furnace,  parallel  with  .the  real  assay.  What- 
ever loss  the  known  amounts  of  precious  metal  in  the  check  sustain 
is  added  to  the  weight  of  metal  obtained  from  the  sample  as  a 
correction,  the  sum  being  supposed  to  represent  the  actual  metal 
present  in  the  sample.  This  method  of  correction  is  always  used 
in  the  assay  of  gold  and  other  precious  metal  bullions,  and  is 
sometimes  used  in  the  assay  of  high-grade  ores.  A  more  detailed 
description  of  the  method  will  be  found  in  the  chapter  on  the  assay 
of  bullion.  This  method,  when  properly  applied,  is  the  better 
and  gives  a  very  close  approximation  to  the  actual  precious  metal 
contents  of  a  sample. 

In  the  case  of  rich  ores  and  furnace  products  other  than  bul- 
lion, a  correction  is  usually  made  by  assaying  the  slags  and  cupels 
resulting  from  the  original  assay.  The  weights  of  gold  and  sil- 
ver thus  recovered  are  added  as  corrections  to  the  weights  first 
obtained.  This  method,  while  approximating  the  actual  con- 
tents of  an  ore,  may  occasionally  give  results  a  little  too  high,  for 
although  gold  and  silver  lost  by  volatilization  is  not  recovered  and 
the  corrections  themselves  must  invariably  suffer  a  second  slag 
and  cupel  loss,  yet  on  the  other  hand,  the  cupeled  metal  from 
both  the  first  and  second  operations  is  not  pure  and  may  retain 
enough  lead  and  occasionally  other  impurities  from  the  ore  and 
extra  litharge  used  to  more  than  offset  the  above  small  losses. 
The  results  of  assays  corrected  by  this  method  are  evidently  some- 
what uncertain,  but  are  nevertheless  much  nearer  to  the  real 
silver  content  than  are  the  results  of  the  uncorrected  or  ordinary 
commercial  assay. 

Smelter  contracts  are  almost  invariably  still  written  on  the 
basis  of  the  ordinary  or  uncorrected  assay  and  when  the  corrected 
assay  is  made  the  basis  of  settlement,  a  deduction  is  made  amount- 
ing to  the  average  correction.  This  amounted  to  1.1  per  cent 
in  the  case  of  certain  Cobalt  ores. 


208  A   TEXTBOOK  OF  FIRE  ASSAYING 

Assay  of  Slags.  —  Assay  slags  are  of  such  variable  composi- 
tion that  no  one  method  af  analysis  is  universally  applicable. 
Almost  any  plan  of  treatment  whereby  the  slag  is  fused  and  a  lead 
button  reduced  will  result  in  the  recovery  of  an  additional  amount 
of  silver,  but  to  make  sure  of  obtaining  practically  all  of  the 
precious  metals  is  quite  another  matter.  Keller*  states  that 
to  obtain  a  full  recovery  of  the  silver  from  slags  it  is  necessary  to 
reduce  practically  all  of  the  lead  from  the  charge  and  it  is  recom- 
mended that  this  procedure  be  followed. 

In  general,  it  is  best  to  have  the  second  slag  differ  materially 
from  the  original  in  order  to  ensure  complete  decomposition.  It 
should  be  noted  that  the  acid  lead  silicates  are  not  decomposed 
by  carbonaceous  reducing  agents,  so  that  the  slags  resulting  from 
Class  1  ores  will  have  to  be  decomposed  by  means  of  metallic 
iron.  Some  additional  borax  may  be  required  as  a  flux  for  the 
ferrous  silicate  resulting  from  the  reaction  of  iron  on  lead  silicate 
and  if  necessary  an  additional  amount  of  sodium  carbonate  may 
be  added. 

In  decomposing  slags  from  niter  assays  by  means  of  iron,  it  is 
advisable  to  carefully  separate  and  reject  the  layer  of  fused  sul- 
phates which  will  be  found  on  top  of  the  cone  of  slag.  If  this  is 
not  done,  the  nails  will  be  greatly  corroded  and  even  cut  in  two 
by  the  reaction  with  the  fused  sulphate;  the  formation  of  iron 
oxide  and  the  production  of  an  alkaline  iron  sulphide  will  result. 
The  reaction  is  probably  as  follows : 

Na2SO4  +  3Fe  =  Na2S  +  Fe3O4. 

If  the  lead  button  obtained  is  too  large  for  cupellation,  as  will 
be  the  case  in  the  decomposition  of  slags  resulting  from  excess 
litharge  fusions,  it  may  be  scorified  to  20  or  25  grams. 

Slags  resulting  from  iron-nail  assays  should  be  fused  with  an 
excess  of  litharge,  to  ensure  decomposing  all  of  the  sulphide  with 
which  the  precious  metals  are  combined.  Borax  and  silica  may 
be  added,  if  necessary,  to  slag  the  resultant  iron  oxide  as  ferrous 
singulo-silicate.  The  slag  resulting  from  an  iron-nail  assay  of 
pure  pyrite  will  probably  contain  about  3.5  grams  of  sulphide 
sulphur.  This  would  reduce  about  90  grams  of  lead  from  an 
excess  of  litharge.  By  limiting  the  amount  of  litharge  it  is  possible 
to  obtain  a  smaller  lead  button,  which  should,  however,  in  this  case, 
collect  practically  all  of  the  gold  and  silver  contained  in  the  slag. 
*  Trans.  A.I.M.E.,  46,  p.  782. 


ASSAY  OF  COMPLEX  ORES  AND  SPECIAL  METHODS    209 

Assay  of  Cupels.  —  Cupel  materials  are  all  refractory,  par- 
ticularly magnesia,  and  for  this  reason  all  unsaturated  cupel 
material  should  always  be  rejected  before  the  cupel  is  pulverized. 
The  student  should  also  bear  in  mind  that  both  Portland  cement 
and  magnesia  are  basic  and  require  the  addition  of  a  considerable 
amount  of  acid  reagents  to  make  a  slag  of  satisfactory  character. 
When  a  corrected  assay  is  to  be  made  the  original  lead  button 
should  not  weigh  more  than  28  grams,  or  too  large  an  amount  of 
cupel  material  will  have  to  be  handled.  The  fluxes  have  to  be 
carefully  proportioned;  and  in  order  to  get  complete  recovery  of 
the  silver  all  of  the  absorbed  litharge  must  be  reduced.  For  this 
reason  it  is  generally  best  not  to  use  any  litharge  flux.  In  the 
charges  which  follow  the  proportions  of  reagents  are  all  based  on 
the  weight  of  bone-ash,  dry  cement  or  magnesia  in  the  material 
being  assayed.  This  may  be  determined,  closely  enough  for 
practical  purposes,  by  calculating  the  weight  of  litharge  corre- 
sponding to  the  lead  button  cupeled,  and  subtracting  this  cal- 
culated weight  from  the  weight  of  saturated  cupel  material. 

BONE-ASH.  —  To  assay  a  bone-ash  cupel,  first  remove  and  reject 
the  unsaturated  part  of  the  cupel,  in  order  to  have  as  little  of  this 
refractory  material  as  possible  to  deal  with.  Weigh  the  saturated 
part,  which  will  be  about  50  per  cent  bone-ash  and  50  per  cent 
litharge  and  grind  to  80-  or  100-mesh  on  a  clean  bucking-board. 

Finally  clean  off  what  sticks  to  the  board  and  muller  by  grind- 
ing a  quantity  of  20-mesh  quartz  equal  to  the  silica  require- 
ments of  the  charge.  For  an  ordinary  assay  this  will  be  about 
10  grams.  This  quartz  is  added  to  the  charge  and  serves  as  a 
flux  for  some  of  the  bases. 

To  flux  bone-ash  add  one  and  a  half  times  its  weight  of  normal 
sodium  carbonate,  two-thirds  of  its  weight  of  borax-glass,  half  its 
weight  of  fluorspar,  and  one-third  its  weight  of  silica.  In  assaying 
a  cupel,  an  excess  of  flour  is  added  to  reduce  all  of  the  litharge. 
For  example,  the  charge  for  a  bone-ash  cupel  would  work  out  as 
follows : 

Cupel  material  (30  grams  bone-ash,  30  grams  litharge)  60  grams 

Sodium  carbonate  45 

Borax-glass  20 

Silica  from  cleaning  the  bucking-board  10 

Fluorspar  15       " 

Flour  4       " 


210  A   TEXTBOOK  OF  FIRE  ASSAYING 

Put  into  a  30-gram  crucible,  mix  and  place  in  a  hot  muffle  so 
that  it  will  fuse  rapidly.  Have  the  atmosphere  neutral  or  slightly 
reducing  and  finally  bring  to  a  light  yellow  heat.  Pour  after  half 
an  hour  at  this  temperature.  The  lead  button  obtained  should 
weigh  almost  as  much  as  the  button  first  cupeled,  i.e.,  the  assay  of 
a  cupel  in  which  a  30-gram  lead  button  was  cupeled  should  yield 
somewhat  more  than  28  grams  of  lead. 

The  slag  will  be  an  almost  colorless,  clear  glass.  The  lead  button 
is  cupeled  and  the  bead  weighed  and  parted. 

CEMENT.  —  To  assay  a  Portland  cement  cupel,  remove  and 
reject  the  unsaturated  part.  Weigh  the  saturated  part,  which 
may  contain  as  little  as  40  per  cent  of  cement,  and  grind  it  to 
80-  or  100-mesh.  Clean  the  bucking-board  and  muller  by  grind- 
ing 15  grams  of  20-mesh  quartz,  and  add  this  to  the  charge,  where 
it  will  serve  as  a  flux  for  some  of  the  bases.  It  must  be  included 
when  considering  the  quantity  of  silica  required  for  the  charge. 

To  flux,  add  twice  as  much  sodium  carbonate  as  there  is  cement, 
an  equal  amount  of  borax-glass  and  twice  as  much  silica.  Add 
4  or  5  grams  of  flour  and  fuse  rapidly  in  a  neutral  or  reducing 
atmosphere.  The  charge  for  a  Portland  cement  cupel  would 
work  out  as  follows : 

Cupel  material  (20  grams  cement,  30  grams  litharge)       50  grams 
Sodium  carbonate  40       " 

Borax-glass  20       " 

Flour  5      " 

Silica  40      " 

This  yields  a  clear-green  glassy  slag  and  a  lead  button  weighing 
about  90  per  cent  as  much  as  the  original  button.  The  silver 
recovery  is  not  as  good  as  that  obtained  from  bone-ash  cupels, 
probably  because  the  lead  recovery  is  not  so  good. 

MAGNESIA.  —  To  assay  a  magnesia  cupel  proceed  as  for  Port- 
land cement.  The  patent  magnesia  cupels  are  the  least  porous 
and  therefore  the  least  of  all  suited  for  corrected  assays,  because 
so  very  much  more  flux  will  have  to  be  provided  for  them.  The 
saturated  part  of  a  magnesia  cupel  is  almost  60  per  cent  magnesia. 
Therefore,  the  quantity  of  magnesia  used  in  absorbing  a  given 
quantity  of  litharge  is  more  than  twice  the  quantity  of  Portland 
cement  required  to  absorb  the  same  quantity  of  litharge.  Con- 
sequently twice  as  much  of  the  reagents  will  have  to  be  used  to 
assay  the  magnesia. 


CHAPTER  X. 
THE  ASSAY  OF  BULLION. 

Bullion,  from  an  assayer's  point  of  view,  is  an  alloy  containing 
enough  of  the  precious  metals  to  pay  for  parting. 

The  different  bullions  are  usually  named  to  correspond  to 
their  major  components,  for  instance,  copper  bullion,  an  alloy 
of  copper  with  small  amounts  of  other  impurities,  as  well  as  some 
gold  and  silver.  In  the  same  way  we  have  lead,  silver  and  gold 
bullions.  Dore  bullion  is  silver  bullion  containing  gold  as  well 
as  a  small  percentage  of  base  metals.  Dore  bars  differ  in  silver 
content  from  600  to  990  parts  per  thousand;  the  base  metals 
consist  chiefly  of  copper,  lead  and  antimony.  The  term  base 
bullion  is  used  in  two  different  senses.  According  to  the  lead 
smelter's  definition,  base  bullion  is  argentiferous  lead,  usually 
the  product  of  the  lead  blast-furnace;  according  to  the  mintman's 
and  refiner's  definition  it  is  bullion  containing  from  10  to  60  per 
cent  of  silver,  usually  some  gold,  and  a  large  percentage  of  base 
metals,  particularly  copper,  lead,  zinc  and  antimony.  Fine  gold 
bars  are  those  which  are  free  from  silver  and  sufficiently  free 
from  other  impurities  to  make  them  fit  for  coinage  and  use  in 
the  arts,  usually  990  to  999  fine. 

The  results  of  lead  and  copper  bullion  assays  are  reported  in 
ounces  per  ton  as  in  the  case  of  ore  assays,  but  in  the  assay  of 
silver,  gold  and  dore  bullions  the  results  are  reported  in  "fineness," 
i.e.,  so  many  parts  of  silver  or  gold  in  one  thousand  parts  of 
bullion.  Thus  sterling  silver  is  925  parts  fine,  that  is  to  say, 
it  is  92.5  per  cent  silver. 

Weights.  —  In  assaying  gold,  silver  and  dore  bullion,  a  special 
set  of  weights,  called  gold-assay  weights,  are  used.  This  is  termed 
the  "  millieme  "  system;  the  unit,  1  millieme,  weighs  0.5  milli- 
gram, and  therefore  the  1000  millieme  weight  equals  0.5  grams. 
Ordinary  weights  in  the  gram  system  may  be  used,  but  as  0.5  gram 
is  the  quantity  of  bullion  commonly  taken  for  assay  the  use  of  the 
millieme  system  saves  computation  in  obtaining  the  fineness. 

211 


212  A   TEXTBOOK  OF  FIRE  ASSAYING 

SAMPLING  BULLION. 

Bullion  may  be  sampled  either  in  the  molten  or  in  the  solid 
condition.  When  it  may  be  melted  and  kept  free  from  dross 
the  dip  or  ladle  sample  is  usually  the  more  accurate  method.  As 
the  weight,  as  well  as  the  assay  of  the  bullion  must  be  known 
in  order  to  value  it,  the  sampling  of  large  lots  of  bullion  by  the 
dip  sample  method  often  presents  difficulties,  owing  to  changes 
in  weight  or  purity  in  the  considerable  length  of  time  necessary 
for  pouring.  Again,  it  is  not  always  convenient  to  melt  a  lot  of 
bullion  to  obtain  a  sample,  and  other  means  must  be  found. 
Sampling  solid  bullion  by  punching,  drilling,  sawing  or  chipping, 
under  certain  conditions,  may  be  made  to  yield  good  results. 
Lead  bullion  is  usually  sampled  by  punching  one  or  more  holes 
in  each  bar,  and  combining  and  melting  the  punchings.  Copper 
bullion  is  now  generally  cast  in  the  form  of  slabs  or  anodes,  and 
these  are  drilled. 

Sampling  Molten  Bullion.  —  The  most  satisfactory  method  of 
sampling  bullion  is  to  melt  the  whole  in  a  suitable  vessel,  stir 
thoroughly  with  a  graphite  rod  or  iron  bar  to  mix  and  then, 
immediately  before  pouring,  ladle  out  a  small  amount  and  granu- 
late it  by  pouring  into  a  pail  of  water.  If  these  operations  are 
correctly  performed  there  is  no  chance  for  segregation,  and  each 
particle  of  the  granulated  metal  should  be  a  true  representa- 
tive of  the  whole.  If  a  granulated  sample  is  not  desired,  a 
ladleful  of  the  mixed  molten  metal  may  be  poured  into  a  thick- 
walled  flat  mold  so  that  it  chills  almost  instantly,  and  a  drill 
or  saw  sample  may  be  taken  from  this.  When  a  ladle  sample  is 
taken,  the  ladle  must  be  so  hot  as  not  to  allow  the  forming  of  any 
solidified  metal  or  "  sculls,"  as  this  would  interfere  with  the 
homogeneity  of  the  sample.  This  method  of  sampling  is  most 
satisfactory  for  bullions  which  do  not  oxidize  or  form  dross  on 
melting,  as  this  of  course,  adds  a  complication  for  which  it  is 
difficult  to  allow. 

Sampling  Solid  Bullion.  —  The  principal  difficulty  encountered 
in  sampling  bullion  in  the  form  of  bars  or  ingots  is  due  to  the 
irregular  distribution  of  the  various  constituents  caused  by  seg- 
regation in  cooling.  If  it  were  possible  to  cool  a  bar  instantly, 
segregation  could  be  prevented,  and  a  chip  or  boring  taken  from 
any  part  would  be  representative.  As  instant  cooling  is  im- 


THE  ASSAY  OF  BULLION 


213 


possible,  the  sampling  of  bars  of  the  ordinary  dimensions  is  usually 
a  difficult  problem.  Occasionally  a  bar  of  bullion  may  be  en- 
tirely homogeneous,  but  this  is  rare;  and  unfortunately  there 
are  no  characteristics  by  which  this  homogeneity  can  be  recog- 
nized. Heterogeneity  is  the  rule,  and  the  explanation  for  this 
common  condition  is  found  in  the  presence  in  almost  every  bullion 
of  constituents  having  different  freezing-points.  In  slow  cool- 
ing, solidification  begins  first  on  the  walls  of  the  mold  and  the 
constituent  having  the  highest  freezing-point  starts  crystallizing 
here,  forcing  the  part  which  is  still  liquid  away  from  the  walls. 


I92J 
0.32 


1/4.5 
0,22 


71.3 
0.24 


195.2 
0.34 


117.0 
0.28 


70.3 
0.22 


194.5 
0.34 


122.3 
0.30 


69.8 
0.22 


122.0 
0.26 


105.1 
0.26 


69.8 
0.22 


68.1 
0.20 


67.2 
0.20 


70.5 
0.22 


FIG.  48.  —  Distribution  of  silver  and  gold  in  a  block  of  blister  copper. 

Solidification  progresses  away  from  the  walls  and  sometimes  also 
away  from  the  surface,  toward  the  center  of  solidification,  at 
which  locus  the  alloy  of  lowest  melting-point  freezes.  This 
naturally  results  in  a  certain  amount  of  migration  of  the  different 
constituents,  toward  or  away  from  the  various  cooling  surfaces 
and  in  a  direction  normal  to  these  surfaces.  According  to  their 
amount,  as  well  as  upon  the  nature  and  amount  of  the  other 
constituents  of  the  alloy,  the  gold  and  silver  may  concentrate 
either  toward  or  away  from  the  center  of  solidification. 

Figure  48  shows  the  distribution  of  silver  and  gold  in  a  block  of 
blister  copper.  To  obtain  these  figures  the  block  was  cut  in 
two,  half  of  the  section  was  laid  off  into  squares  as  indicated  and 
a  sample  was  taken  by  drilling  a  hole  in  the  center  of  each  square. 
The  upper  figure  in  each  case  represents  the  silver  assay  in  ounces 


214 


A    TEXTBOOK  OF  FIRE  ASSAYING 


per  ton  and  the  lower  one  gold  in  ounces  per  ton.  In  this 
case,  the  precious  metals,  particularly  the  silver,  have  con- 
centrated toward  the  center  of  solidification,  which  is  slightly 
above  the  geometrical  center  of  the  solid.  It  is  obviously  next 
to  impossible  to  locate  a  drill-hole  which  would  take  a  representa- 
tive sample  of  such  a  block,  and  no  chip  taken  from  a  corner 
could  possibly  give  anything  like  the  truth.  A  saw-section 
through  the  center  would  probably  be  satisfactory,  provided  the 
entire  amount  of  sawings  were  assayed.  Figure  49  shows  another 
example  of  the  distribution  of  the  precious  metals  in  copper 


^v___ 

/~ 

270.4 
2.80 

322.0 

3.08 

328.0 
3.16 

341.4 
3.24 

362.  8 
3.32 

309.4 
3,08 

338.4 

3.24 

360.2 
3.32 

357.0 
3.28 

368.6 
3.36 

350.0 
3.36 

351.8 
3.32 

358.0 
3.36 

353,4 
3.32 

364.6 
3.28 

353.0 
3.32 

363.4 

3.40 

366+2 

3.40 

365.2 

3.40 

'360.0 
3.36 

Fig.  49.  —  Distribution  of  silver  and  gold  in  a  block  of  refined  copper. 

bullion;  but  in  this  case  the  concentration  was  in  the  opposite 
direction,  i.e.,  toward  the  part  which  solidified  first.  The  same 
thing  which  is  illustrated  here  for  copper  is  true  to  a  lesser  extent 
for  lead  bullion  and  for  impure  precious  metal  bullions,  and  in 
these  cases  too  the  concentration  may  be  either  away  from  or 
toward  the  center  of  solidification. 

The  amount  of  this  diffusion  or  segregation  is  dependent  upon 
a  number  of  factors,  the  most  important  of  which  are  the  composi- 
tion of  the  alloy  and  the  rate  of  cooling.  The  shape  and  thick- 
ness of  the  mold,  as  well  as  its  initial  temperature  and  the  tem- 
perature of  the  alloy  when  poured,  are  also  important  factors 
in  this  problem. 

It  has  been  conclusively  demonstrated  that  it  is  impossible 
to  obtain  samples  of  sufficient  accuracy  from  copper  bars  or 


THE  ASSAY   OF  BULLION 


215 


pigs  ot  the  usual  dimensions,  except  by  sawing,  which  is  entirely 
too  expensive  a  proceeding  for  everyday  use. 

To  eliminate  the  difficulty  of  sampling  from  a  bar,  Keller* 
recommends  casting  the  metal  in  a  thin  plate  or  slab,  and  this 
practice  has  now  been  almost  universally  adopted  by  the  copper 
smelters.  The  slabs  are  usually  made  some  30  or  40  inches 
square  and  only  1  or  2  inches  thick.  Of  course,  some  concentra- 
tion takes  place  here,  also,  but  as  the  plate  solidifies  so  much  fas- 
ter than  the  same  metal  cast  in  a  bar  or  ingot  this  factor  has  less 
weight. 


FIG.  50.  —  Diagrammatic  section  through  a  plate  of  metal  illustrating  direction 
of  segregation   towards  or  away  from    center  of  solidification. 

Figure  50  is  an  ideal  section  through  a  part  of  such  a  slab.  The 
concentric  lines  indicate  the  progressive  cooling  toward  the  center 
of  cooling.  It  may  be  assumed  that  solidification  progresses  so 
as  to  form  even  layers  from  all  the  surface  planes  of  the  slab  and 
that  each  successive  layer  differs  in  composition  from  its  prede- 
cessor. On  the  right-hand  side  of  the  figure,  just  beyond  the 
center  of  solidification,  is  shown  a  region,  not  wider  than  the 
thickness  of  the  plate,  where  concentration  has  taken  place  both 
horizontally  and  vertically.  All  around  the  slab  there  will  be  a 
strip  like  this.  Inside  of  this  strip,  the  width  of  which  is  the 
same  as  the  thickness  of  the  slab,  there  can  be  movement  only 
in  a  vertical  plane.  Therefore,  the  solid  constituting  this  strip 
contains  in  its  entirety  a  fair  proportion  of  all  the  constitutents 
*  Trans.  A.I.M.E.,  27,  p.  106. 


216  A    TEXTBOOK  OF  FIRE  ASSAYING 

of  the  alloy,  but  it  is  impossible  to  sample  this  correctly.  The 
solid  inside  of  this  strip  also  contains  a  fair  proportion  of  all  the 
constituents  of  the  alloy,  and  as  here  there  is  concentration  in 
the  vertical  direction  only,  a  hole  drilled  through  the  plate  any- 
where should  yield  a  correct  sample  of  the  whole.  The  method 
advocated  by  Keller  has  been  demonstrated  to  yield  satisfactory 
results  and  has  now  become  standard  in  the  copper  industry. 

Some  typical  methods  of  sampling  lead  and  copper  bullion 
follow. 

Sampling  Lead  Bullion.  —  Lead  bullion  is  sampled  both  in 
the  liquid  and  in  the  solid  state.  In  either  case  it  is  now  cus- 
tomary to  transfer  the  lead  from  the  blast-furnace  either  into  a 
reverberatory  furnace  or  into  large  kettles  holding  20  to  30  tons. 
Here  the  lead  is  purified  by  cooling  to  a  little  above  the  melting- 
point  of  pure  lead.  By  doing  this,  a  large  part  of  the  impurities 
which  are  held  in  solution  by  the  superheated  lead  separate  out 
as  a  dross  which  is  carefully  removed  by  skimming.  The  re- 
maining lead,  now  in  a  better  condition  to  sample,  is  drawn  off 
and  cast  into  bars  of  about  100  pounds. 

In  taking  a  dip  sample,  a  small  ladleful  is  taken  at  regular  in- 
tervals from  the  stream  coming  from  the  drossing  kettle.  These 
individual  samples  are  carefully  remelted  at  a  dark  red  heat  in  a 
graphite  crucible,  the  melt  is  well  stirred  and  cast  in  a  heavy- 
walled  shallow  mold,  making  a  cake  about  10  inches  long,  5 
inches  wide  and  i  inch  thick.  This  cools  so  quickly  that  there  is 
little  or  no  chance  for  segregation.  The  final  assay  samples  are 
taken  from  this  cake  by  sawing  and  taking  the  sawdust,  or  by 
boring  entirely  through  the  slab  in  a  number  of  places,  and  taking 
the  borings,  or  by  cutting  out  four  or  more  0.5  assay-ton  pieces 
from  different  parts  of  the  bar  and  using  these  directly. 

Another  and  more  modern  method  of  sampling  lead  bullion, 
which  does  away  with  the  remelting,  is  to  take  a  number  of  dip 
samples  in  the  shape  of  gum-drops.  While  the  material  in  the 
kettle  is  being  thoroughly  stirred,  the  mold,  which  has  six  or  eight 
conical  depressions  and  is  provided  with  a  long  handle,  is  inserted 
and  heated  to  the  same  temperature  as  the  molten  metal.  The 
"  gum-drops  "  are  dipped  out  and  cooled  in  the  mold,  by  dipping 
the  bottom  of  the  latter  into  water.  These  "  gum-drops  "  which 
weigh  from  15  to  25  grams,  are  weighed  without  clipping  and 
cupeled,  and  the  results  are  computed. 


THE  ASSAY  OF  BULLION  217 

Bars  of  solid  lead  bullion  are  sampled  by  means  of  a  heavy 
punch  which  takes  a  cylindrical  sample  about  2  inches  long  and 
|  inch  in  diameter.  There  are  naturally  a  number  of  different 
systems,  but  the  most  common  method  is  to  place  five  bars  side 
by  side  and  face  up,  and  punch  a  hole  in  each  extending  halfway 
through.  Each  bar  is  punched  in  a  different  place  and  in  such 
a  way  that  the  holes  make  a  diagonal  across  the  five  bars.  The 
bars  are  then  turned  over  and  another  sample  is  taken  from  each 
along  the  opposite  diagonal.  Usually  a  carload  of  about  20  or 
30  tons  is  sampled  as  one  lot.  The  punchings  from  such  a  lot, 
weighing  from  8  to  15  pounds,  are  melted  in  a  graphite  crucible 
and  cast  into  a  flat  bar,  from  which  the  final  assay  samples  are 
taken  by  sawing,  drilling  or  cutting. 

Sampling  Copper  Bullion.  —  The  sampling  of  copper  bullion 
may  be  classified  into  smelter  methods,  and  refinery  methods. 
The  bullion  is  quite  universally  cast  in  the  form  of  anodes  at 
the  smelter,  and  shipped  to  the  refinery  in  this  form.  This 
renders  remelting  at  the  refinery  unnecessary,  and  the  result  is 
that  the  refiners  sample  the  solid  bullion  by  drilling.  The  smel- 
ters, having  the  bullion  in  the  molten  condition,  generally  sample 
it  in  this  condition  on  account  of  the  greater  ease  and  less  ex- 
pense. 

Probably  the  most  satisfactory  smelter  method  of  sampling 
is  the  "  splash-shot  method/'  which  consists  in  shotting  into 
water  a  small  portion  of  the  molten  stream  of  copper  as  it  flows 
from  the  refining  furnace,  by  "  batting  "  the  stream  with  a  wet 
stick.  This  operation  is  repeated  at  uniform  intervals  during 
the  pouring,  the  amount  taken  each  time  being  kept  about  the 
same.  The  samples  are  dried  and  dirt  and  pieces  of  burned 
wood  are  removed.  All  material  over  4-mesh  and  under  10-mesh 
is  rejected,  and  the  remainder  taken  as  the  sample.  This  method, 
when  properly  carried  out,  gives  results  which  check  within  prac- 
tical limits  with  the  drill  sample  of  the  anodes  taken  at  the  refinery. 

Another  method  which  is  used  to  some  extent  for  sampling 
molten  copper  bullion  is  known  as  the  "  ladle-shot  method." 
This  consists  in  taking  a  ladleful  from  the  furnace  or  from  the 
stream  of  the  casting  machine  and  shotting  it  by  pouring  over  a 
wooden  paddle  into  water.  In  this  method  at  least  three  ladlefuls 
are  taken,  one  near  the  beginning,  one  at  the  middle,  and  one 
near  the  end  of  the  pour.  The  shots  are  treated  in  the  same 


218  A    TEXTBOOK  OF  FIRE  ASSAYING 

manner  as  before.  This  method  is  not  thought  so  well  of  as 
the  previous  one  on  account  of  segregation  toward  or  from  the 
"  sculls  "  which  are  left  in  the  ladles. 

Instead  of  shotting  and  taking  the  shot  for  the  final  sample, 
W.  H.  Howard  of  Garfield,  Utah,  recommends  ladling  into  a 
flat  disc.  This  "  pie  sample  "  is  sawed  radially  a  number  of 
imes,  and  the  sawdust  is  used  for  the  final  sample. 

The  following  description  of  the  method  of  sampling  anodes 
at  Perth  Amboy,  N.  J.,  is  typical  of  refinery  methods  of  sampling 
and  is  the  method  developed  by  Dr.  Edward  Keller.  The  copper 
is  received  in  the  form  of  anodes  36  inches  long,  28  inches  wide 
and  2  inches  thick.  These  are  carefully  swept  to  remove  foreign 
matter,  and  then  drilled  with  a  0.5  inch  drill  completely  through 
the  anode,  all  of  the  drillings  being  carefully  saved.  A  99-hole 
template  is  used  to  locate  the  holes  which  are  spaced  3TV  inches 
center  to  center,  and  the  outside  row  is  approximately  2J  inches 
from  the  edge  of  the  anode.  The  holes  of  the  template  are  used 
in  continuous  order,  one  hole  to  the  anode. 

For  very  rich  anodes  some  refiners  use  a  template  having  as 
many  as  240  holes,  but  it  seems  doubtful  if  this  arrangement  of 
spacing  a  single  hole  in  each  anode  will  yield  any  better  sample. 

With  low-grade,  uniform  bullion  every  fourth  anode  only  is 
drilled.  A  30-ton  lot  of  anodes  in  which  each  one  is  drilled  will 
yield  6  or  8  pounds  of  drillings,  which  are  ground  in  a  drug-mill 
fitted  with  manganese  steel  plates  and  reduced  by  quartering  to 
about  2  pounds.  This  sample  is  reground  until  it  will  all  pass 
a  16-mesh  screen,  and  is  then  divided  into  the  sample  packages. 

Sampling  Dor6  Bullion.  —  Dore  bullion  is  sampled  in  the  mol- 
ten state  by  dip-sampling  and  in  the  solid  state  by  drilling. 

The  dore  bullion  at  one  plant  is  cast  into  plates  18  inches  by 
7  inches  by  f  inch  and  sampled  by  drilling  ¥9T  inch  holes  in  it,  on 
the  checker-board  plan.  The  drillings  are  ground  to  pass  a 
30-mesh  screen.  An  electromagnet  is  used  to  remove  from  the 
sample  all  the  iron  from  the  drills  and  mill. 

Sampling  Gold  Bullion  (United  States  Mint  Method).  —  Every 
lot  of  bullion  or  dust  receive^  at  any  United  States  Assay  Office 
or  Mint  is  immediately  weighed  and  given  a  number.  It  is  then 
melted  in  a  graphite  crucible  with  borax  to  make  the  deposit 
uniform,  and  cast  into  a  bar  whose  horizontal  dimensions  are 
approximately  12J  inches  by  5J  inches.  Usually  no  attempt 


THE  ASSAY  OF  BULLION  219 

is  made  to  refine  it  unless  it  is  very  impure.  Occasionally,  in  the 
case  of  very  impure  bullions,  a  small  dip  sample  is  taken  and  gran- 
ulated, but  in  general  the  whole  melt  is  cast  and  sampled  as 
noted  below.  The  slag  is  poured  with  the  bar  and  when  solid 
is  ground  and  panned,  and  the  recovered  prills  are  dried,  weighed 
and  allowed  for  in  computing  the  value  of  the  bar. 

After  the  bar  is  cleaned  of  slag  it  is  dried,  weighed  and  num- 
bered, and  if  it  is  thought  to  be  homogeneous,  two  samples  of 
3  or  4  grams  each  are  chipped  from  diagonally  opposite  corners. 
These  are  flattened  with  a  heavy  hammer,  annealed  and  rolled 
into  sheets  thin  enough  to  be  easily  cut  with  shears.  The  use 
of  the  shears  can  only  be  learned  by  practice,  but  assayers  be- 
come very  skilful  after  a  time,  and  it  is  no  unusual  thing  to  see 
a  bullion  assayer  weigh  out  five  samples  in  almost  as  many  min- 
utes. 

Cyanide  bars,  which  do  not  give  checks  from  chipped  samples, 
are  drilled  halfway  through  on  two  opposite  corners  of  the  top 
at  a  point  about  1  inch  from  each  edge.  These  drillings  are  mixed 
and  assayed  as  the  top  sample.  The  bottom  sample  is  taken  in 
the  same  manner,  except  that  the  drilling  is  done  on  the  other 
two  corners.  The  top  and  bottom  samples  are  kept  separate. 

THE  ASSAY   OF  LEAD   BULLION. 

A  description  of  the  cupellation  assay  of  lead  bullion  has  al- 
ready been  given  in  the  chapter  on  cupellation.  In  smelter  con- 
trol work  the  assay  is  usually  made  in  quadruplicate.  If  the 
bullion  contains  sufficient  copper,  arsenic,  antimony,  tin  or  other 
base  metals  to  influence  the  results  of  the  cupellation  assay, 
three  or  four  portions  of  0.5  or  1.0  assay-ton  are  scorified  with 
the  addition  of  lead  until  the  impurities  are  eliminated,  when 
the  resultant  buttons  are  cupeled. 

CORRECTION  FOR  CUPEL  Loss.  —  In  some  instances  the  slags 
and  cupels  are  reassayed  and  the  weight  of  the  gold  and  silver 
found  is  added  to  that  obtained  from  the  first  cupellation.  There 
is  no  fixed  custom  as  yet  regarding  the  use  of  corrected  assays. 
In  most  of  the  custom  smelters,  the  uncorrected  assay  is  used  as 
the  basis  of  settlement;  but  some  of  the  large  concerns  who  have 
their  own  refineries  are  using  the  corrected  assay  in  their  inter- 
plant  business. 


220  A   TEXTBOOK  OF  FIRE  ASSAYING 

THE  ASSAY  OF  COPPER  BULLION. 

Copper  bullion  may  be  assayed  by  the  scorification  method, 
by  the  crucible  method  or  by  a  combination  of  wet-and-fire  meth- 
ods. In  the  combination  method  the  bullion  is  treated  with  sul- 
phuric or  nitric  acid  which  dissolves  the  copper  and  more  or  less 
of  the  silver  but  leaves  the  gold.  The  silver  is  precipitated  by 
suitable  reagents  and  filtered  off  together  with  the  gold.  The 
filter  paper  and  contents  are  put  into  a  scorifier  or  crucible  with 
reagents  and  the  assay  finished  by  fire  methods. 

The  Scorification  Method.  —  The  following  method,  commonly 
referred  to  as  the  "  all-fire  "  method  is  a  modification  kindly 
supplied  by  Mr.  H.  D.  Greenwood,  Chief  Chemist  for  the  United 
States  Metals  Refining  Co.,  Chrome,  N.  J. 

Sample  down  the  finely  ground  bullion  on  a  split  sampler  in 
such  a  way  as  to  obtain  a  sample  of  about  1  assay-ton  which  will 
include  the  proper  proportion  of  the  finer  and  the  coarser  parts  of 
the  borings.  This  sampling  must  be  conducted  carefully,  as  the 
precious  metal  content  of  the  finer  portion  differs  somewhat 
from  that  of  the  coarser  portion  of  the  sample.  Portions  "  dipped  " 
from  the  sample  bottle  or  from  the  sample  spread  out  on  paper 
are  likely  to  contain  undue  amounts  of  coarse  or  of  fine. 

Weigh  out  four  portions  of  copper  borings  of  0.25  assay-ton 
each,  mix  with  50  grams  test  lead,  put  in  3-inch  Bartlett  scorifiers, 
cover  with  40  grams  test  lead  and  add  about  1  gram  SiO2.  Scor- 
ify hot,  heating  at  the  end  so  that  they  will  pour  properly.  Add 
test  lead  to  make  weight  of  buttons  plus  test  lead  equal  to  70 
grams,  add  1  gram  SiO2  and  scorify  rather  cool.  Pour,  make  up 
to  60  grams  with  test  lead,  adding  1  gram  SiO2  and  scorify  again. 

Combine  the  buttons  two  and  two,  and  make  up  each  lot  to 
85  grams  with  test  lead,  adding  1  gram  SiO2,  and  scorify  very  cool. 
Make  up  buttons  to  70  grams  by  adding  test  lead,  add  1  gram 
SiO2  and  scorify  for  the  fifth  time.  The  buttons  should  be  free 
from  slag  and  weigh  14  grams. 

Cupel  at  a  temperature  to  feather  nicely,  and  raise  the  heat  at 
the  finish.  Cupels  should  be  made  of  60-mesh  bone-ash,  and  should 
be  of  medium  hardness. 

Weigh  the  beads  and  part  as  usual.  Dry,  anneal  and  weigh 
the  gold.  The  two  results  should  check  within  .02  ounce  per 
ton,  and  the  average  figure  is  to  be  reported.  If  the  silver 


THE  ASSAY  OF  BULLION  221 

contents  of  the  bullion  is  low,  add  enough  fine  silver  before  the 
first  scorification  to  make  the  total  silver  in  the  mixture  equal  to 
about  eight  times  the  amount  of  gold. 

The  scorification  method  was  until  recently  accepted  as  stand- 
ard for  gold  and  most  smelter  contracts  involving  this  material 
stated  that  "  gold  shall  be  determined  by  the  all-fire  method  or 
its  equivalent."  The  silver  results  obtained  by  the  scorification 
method  are  not  acceptable,  owing  to  the  considerable  slag  and  * 
cupellation  losses  which  average  perhaps  as  much  as  5  or  10  per 
cent.  Reassay  of  the  slag  and  cupels  will  permit  recovery  of 
most  of  the  silver  and  approximately  1  per  cent  additional  gold. 
The  scorification  assay  is  expensive  as  regards  both  time  and 
material,  and  is  falling  into  disfavor. 

The  Crucible  Method.  —  The  crucible  method  for  gold  and 
silver  in  copper  bullion  was  first  described  by  Perkins*  and  as 
described  by  him  showed  no  great  advantage  over  the  scorifica- 
tion method  as  to  saving  in  time,  cost  of  materials,  or  increased 
furnace  capacity.  The  following  modified  procedure  requires 
about  one-third  of  the  materials,  time  and  furnace  capacity 
necessary  for  that  described  by  Perkins,  and  yet  gives  buttons 
sufficiently  free  from  copper  to  be  cupeled  directly. 

Sample  down  the  finely  ground  bullion  to  about  0.25  assay-ton 
and  adjust  the  weight  of  the  sampled  portion  to  exactly  0.25 
assay-ton.  Place  in  a  20-gram  crucible  and  mix  with  it  1.2  grams 
of  powdered  sulphur.  Cover  this  with  a  mixture  of  15  grams  of 
sodium  carbonate,  240  grams  of  litharge,  and  8  grams  of  silica; 
but  do  not  mix  with  the  sulphur  and  copper,  which  should  be 
allowed  to  remain  in  the  bottom  of  the  crucible.  Cover  with  salt 
or  flux  mixture  and  place  in  a  hot  muffle  so  that  the  charge  will 
begin  to  melt  in  six  or  eight  minutes.  The  fusions  should  be 
quiet  and  ready  to  pour  in  twenty-five  or  thirty  minutes. 

If  a  salt  cover  is  used  the  lead  buttons  should  weigh  about 
32  grams;  -if  a  flux  cover  is  used  they  may  be  somewhat  smaller. 
With  a  properly  conducted  assay  the  buttons  are  soft  enough 
for  direct  cupellation;  but  the  cupels  are  quite  green.  If  the 
assayer  prefers,  the  buttons  may  be  made  up  to  50  or  60  grams 
with  test  lead  and  scorified  in  a  3-inch  scorifier  to  further  elimin- 
ate the  copper.  After  cupellation  the  beads  are  weighed  and 

*  An  "All-Fire"  Method  for  the  Assay  of  Gold  and  Silver  in  Blister 
Copper,  W.  G.  Perkins,  Trans.  A.I.M.E.,  33,  p.  670. 


222  A   TEXTBOOK  OF  FIRE  ASSAYING 

parted  as  usual.  It  is  well  to  make  four  fusions,  and  to  combine 
the  beads,  two  and  two,  for  parting. 

REMARKS.  —  As  soon  as  the  sulphur  melts  it  combines  with 
the  copper  to  form  a  matte.  This  matte  is  later  decomposed 
and  most  of  its  copper  is  oxidized  and  slagged  by  the  litharge 
of  the  charge.  The  fusions  melt  down  very  quietly,  almost 
without  boiling,  and  with  a  short  period  of  fusion  the  crucibles 
are  not  badly  attacked.  The  final  temperature  need  not  be 
higher  than  a  good  bright  red  or  full  yellow.  The  slag  is  heavy 
but  very  fluid,  and  should  not  contain  any  lead  shot. 

The  method  gives  results  in  gold  equal  to  the  scorification 
method;  but,  as  in  any  method  using  high  litharge,  the  silver  is 
apt  to  be  somewhat  low. 

Nitric  Acid  Combination  Method.*  —  Sample  down  the  finely 
ground  bullion  on  a  split  sampler  in  such  a  way  as  to  obtain  a 
sample  of  about  1  assay-ton  which  will  include  the  proper  propor- 
tion of  the  finer  and  coarser  parts  of  the  borings.  This  sampling 
must  be  conducted  carefully  as  the  precious  metal  content  of  the 
finer  portion  differs  somewhat  from  that  of  the  coarser  portion 
of  the  sample.  Portions  "  dipped  "  from  the  sample  bottle  or 
from  the  sample  spread  out  on  paper  are  likely  to  contain  undue 
amounts  of  coarse  or  of  fine. 

Weigh  out  two  portions  of  copper  borings  of  1  assay-ton  each, 
and  carry  the  assay  through,  on  each  portion,  as  follows: 

Place  in  a  No.  5  beaker,  add  100  c.c.  of  distilled  water  and 
90  c.c.  HNO3,  sp.  gr.  1.42,  the  latter  being  added  in  portions  of 
30  c.c.  each,  at  intervals  of  about  one  hour.  When  all  is  in  solu- 
tion, precipitate  a  small  amount  of  silver  chloride  with  salt  solu- 
tion in  order  to  collect  the  gold,  filter  through  double  filter  papers 
and  wash  the  filter  papers  free  from  copper.  To  the  filtrate  add 
the  calculated  amount  of  salt  solution  to  precipitate  all  the  silver 
and  a  slight  excess,  measuring  the  solution  with  a  burette  and 
varying  the  amount  added  with  the  richness  of  the  bullion.  Allow 
to  stand  over  night  after  stirring  well.  Filter  the  silver  chloride 
through  double  papers,  wash  papers  free  from  copper,  then 
sprinkle  5  grams  of  test  lead  in  the  filter  paper  and  fold  into  a 
2J-inch  Bartlett-shape  scorifier,  the  bottom  of  which  is  lined 
with  sheet  lead.  To  this  add  also  the  filter  papers  containing 

*  Procedure  kindly  supplied  by  Mr.  D.  H.  Greenwood,  Chief  Chemist, 
for  the  United  States  Metals  Refining  Company,  Chrome,  N.  J. 


THE  ASSAY  OF  BULLION  223 

the  gold.  Dry  and  ignite  the  filter  papers  carerully,  cover  with 
35  grams  of  test  lead  and  a  little  borax-glass,  and  scorify  at  a 
low  heat  so  that  the  resultant  button  will  weigh  about  12  grams. 
Cupels  should  be  feathered  nicely.  Cupels  should  be  made  of 
60-mesh  bone-ash  and  should  be  of  medium  hardness.  Weigh 
the  bead  and  part.  Anneal  and  weigh  the  gold.  The  two  results 
on  gold  should  check  within  0.02  ounce  per  ton,  and  the  silver 
within  1  per  cent. 

The  nitric  acid  combination  method  has  for  a  long  time  been 
the  standard  for  the  determination  of  silver  in  copper  bullion. 
In  laboratories  where  many  such  determinations  are  made,  a 
number  of  most  ingenious  labor-saving  devices  have  been  devel- 
oped. For  a  description  of  these  the  student  is  referred  to  two 
papers*  by  Edward  Keller. 

The  nitric  acid  combination  method  is  recognized  as  giving 
low  results  in  gold.  Van  Liewf  attributes  this  to  the  solution 
of  the  gold  in  the  mixture  of  nitrous  and  nitric  acids  present. 
He  found  a  loss  of  33.7  per  cent  of  gold,  on  treating  gold  leaf  with 
a  mixture  of  nitrous  and  nitric  acids  for  two  and  a  half  hours. 
He  gives  a  method  of  slow  solution  in  cold  dilute  acid  which  re- 
duces this  loss  to  a  minimum. 

Various  attempts  to  overcome  this  difficulty  have  been  made 
but  none  have  been  completely  successful. 

The  unconnected  silver  results  obtained  by  this  method  are 
from  1.5  to  4  per  cent  low,  according  to  the  amount  of  silver 
contained,  and  unless  this  loss  is  taken  into  account,  it  is  certain 
to  cause  a  great  deal  of  uncertainty  in  the  statistics  of  the  smelt- 
ing industry. 

Keller!  recommends  the  following  method  for  determining  the 
slag  and  cupel  loss.  The  slag  and  cupels  are  crushed,  ground 
and  thoroughly  mixed.  The  whole  or  an  aliquot  part  is  fused 
in  G  crucibles  with  the  following  charge : 

Slag  and  cupels 200  grams 

Sodium  carbonate 70      " 

Borax 70      " 

Flour 10      " 

*  Labor-saving  Devices  in  the  Works  Laboratory,  Trans.  A.I.M.E.,  36,  p. 
3  (1906);  41,  p.  786(1910). 

t  Eng.  and  Min.  Jour.  69,  p.  496  et  seq. 

J  Recent  American  Progress  in  the  Assay  of  Copper  Bullion,  Trans. 
A.I.M.E.,  46,  p.  782. 


224  A   TEXTBOOK  OF  FIRE  ASSAYING 

The  resulting  lead  buttons  are  scorified  and  cupeled.  Keller 
states  that  it  is  necessary  to  reduce  practically  all  of  the  lead  in 
the  slag  and  cupels  in  order  to  obtain  full  recovery  of  the  silver 
and  gold. 

Mercury-Sulphur  Acid  Method.  —  The  copper  bullion  sample, 
,  which  has  been  ground  to  pass  a  16-mesh  screen,  is  first  separated 
into  two  portions  by  means  of  a  40-mesh  screen,  and  each  por- 
tion is  weighed.  As  the  precious-metal  content  of  the  fine  differs 
somewhat  from  that  of  the  coarse  portion,  it  is  important  to  in- 
clude a  proper  proportion  of  each  in  the  sample  taken  for  assay. 
Calling  "  C  "  the  weight  of  the  coarse  and  "  F  "  the  weight  of 

29.166 
the  fine,  weigh  out   F          grams  of  coarse  and  make  up  the  re- 

C~ 

mainder  of  the  assay-ton  with  fine.  Transfer  to  an  800  c.c. 
beaker,  add  30  c.c.  of  water  and  10  c.c.  of  mercury  nitrate  solu- 
tion (Hg  0.25g).  Shake  the  beaker  until  the  copper  is  thoroughly 
amalgamated,  then  add  100  c.c.  of  strong  sulphuric  acid,  cover 
the  beaker  and  place  on  the  hot  plate  and  heat  until  the  copper 
is  all  dissolved.  This  will  take  from  one  to  two  hours  accord- 
ing to  the  temperature  and  the  state  of  division  of  the  sample. 
The  apparent  boiling  of  the  liquid  during  this  time  is  only  bubbling 
and  is  due  to  the  evolution  of  sulphur  dioxide  gas.  This  com- 
pleted, the  supernatant  liquid  assumes  a  dark  green  color,  finally 
changing  to  a  light  grayish-blue,  which  is  the  indication  of  the 
finishing  point. 

Remove  the  beaker  and  allow  to  cool.  The  contents  will 
be  a  semi-liquid  sludge.  When  this  is  cool,  add  about  100  c.c.  of 
cold  water  and  mix,  then  add  400  c.c.  of  boiling  water  and  stir 
until  the  copper  sulphate  dissolves.  Add  sufficient  salt  solution 
to  precipitate  all  of  the  mercury  and  silver  present.  With  100 
milligrams  of  silver  and  0.25  grams  of  mercury,  30  c.c.  of  a  solu- 
tion containing  19  grams  pf  NaCl  per  liter  is  sufficient.  Any 
material  excess  should  be  avoided. 

Boil  the  solution  to  coagulate  the  silver  chloride,  remove  from 
the  hot-plate,  dilute  to  600  c.c.  with  cold  water  and  allow  to 
cool.  Filter  through  double  filter  papers,  wash  the  beaker  and 
filter  with  hot  water.  Finally  wipe  the  inside  of  the  beaker  with 
filter  paper  and  add  this  to  the  material  in  the  filter.  Thorough 
washing  of  the  filter  is  not  necessary. 


THE  ASSAY  OF  BULLION  225 

Transfer  the  wet  filter  and  its  contents  to  a  24-inch  scorifier 
which  has  been  glazed  on  the  inside  by  melting  litharge  in  it 
and  pouring  away  the  excess. 

Burn  off  the  filter  paper  at  a  low  temperature,  preferably  in  a 
closed  oven  which  may  be  heated  to,  say  175°  C.  This  chars  the 
paper  slowly  without  danger  of  loss  of  silver. 

When  the  paper  is  consumed,  add  30  grams  of  test  lead  and 
scorify;  pour  so  as  to  obtain  a  12-gram  button,  cupel  as  usual 
to  produce  feather  litharge,  weigh  the  gold  and  silver  bead  and 
part  with  dilute  nitric  acid. 

The  mercury  solution  mentioned  above  is  made  by  dissolving 
32.5  grams  of  mercury  nitrate  in  a  liter  of  water.  This  makes  a 
solution  containing  approximately  25  grams  of  mercury  per  liter. 
It  should  be  noted  that  with  comparatively  pure  copper  the 
amount  of  mercury  nitrate  may  be  reduced,  while  with  copper 
high  in  sulphur  an  increase  in  the  amount  of  mercury  nitrate 
will  be  required. 

The  object  of  adding  mercury  is  to  secure  an  easy  solution  of 
the  copper  in  sulphuric  acid.  If  the  copper  is  treated  directly 
without  previous  amalgamation,  it  is  very  difficult  to  dissolve 
it  in  sulphuric  acid.  In  fact  a  considerable  portion  of  it  will 
remain  insoluble,  partly  in  the  form  of  sulphide  of  copper.  If, 
on  the  other  hand,  the  copper  be  amalgamated,  solution  pro- 
ceeds smoothly  until  practically  all  of  the  copper  is  dissolved. 
When  the  bullion  is  low  in  precious  metals,  say  less  than  50  ounces 
per  ton,  no  silver  dissolves  in  the  sulphuric  acict.  No  gold  dis- 
solves whatever  the  grade.  If  the  bullion  is  very  rich  in  silver 
a  little  of  the  Jatter  may  dissolve  in  the  acid. 

The  assays  should  be  made  in  duplicate  or  triplicate,  and 
the  average  results  reported.  Differences  in  silver  seldom  ex- 
ceed 0.2  ounce;  the  gold  results  are  usually  exactly  the  same. 
The  sulphuric  acid  used  should  be  chemically  pure  and  full 
strength  (1.84  sp.  gr.). 

The  mercury-sulphuric  acid  combination  method  gives  silver 
results  equal  to  the  nitric  acid  combination  method  and  superior 
to  the  all-fire,  or  scorification  method.  When  the  scorification 
and  cupellation  losses  of  each  method  are  taken  into  account  the 
gold  results  obtained  by  the  mercury-sulphuric  acid  and  the  scor- 
ification methods  are  substantially  identical.  The  mercury- 
sulphuric  acid  combination  method  is  now  generally  accepted 


226  A    TEXTBOOK  OF  FIRE  ASSAYING 

as  standard  for  gold  and  is  fast  coming  to  be  considered  standard 
for  silver  as  well. 

If  the  copper  is  not  all  dissolved,  as  is  sometimes  the  case, 
particularly  with  very  impure  bullion,  this  method  may  give  high 
silver  results,  due  to  the  possibility  of  some  copper  being  re- 
tained in  the  silver  bead. 

THE   ASSAY   OF  DORE  BULLION. 

This  method  is  the  one  generally  adopted  by  assayers  in  this 
country,  and  may  also  be  used  for  the  assay  of  silver  bullion.  A 
better  method  for  the  accurate  determination  of  silver  in  dore 
or  silver  bullion  is  probably  the  Gay-Lussac  or  salt  titration, 
also  known  as  the  mint  method.  This  later  method  requires 
considerable  equipment  and  preparation,  and  for  this  reason  the 
occasional  assay  is  more  easily  performed  by  fire  methods. 

The  Check.  —  In  order  to  correct  for  the  inevitable  losses  in 
cupeling  as  well  as  for  any  other  errors  in  the  assay,  silver,  dore, 
and  gold  bullions  are  always  run  with  a  check.  This  check  or 
"  proof  center  "  is  a  synthetic  sample  made  up  of  known  weights 
of  pure  silver,  gold  and  copper,  to  approximate  as  closely  as 
possible  the  composition  of  the  bullion  to  be  assayed.  It  is 
cupeled  at  the  same  time  and  under  the  same  conditions  as  the 
regular  assays,  and  whatever  gain  or  loss  it  suffers  is  added  as  a 
correction  to  the  regular  assay.  To  obtain  data  to  make  up  the 
check  a  preliminary  assay  is  made.  This  gives  the  approximate 
composition  of  the  bullion. 

Preliminary  Assay.  —  A  sample  of  500  milligrams  of  bullion, 
or  as  nearly  this  amount  as  possible,  is  weighed  out  on  the  assay 
balance,  and  the  exact  weight  recorded.  This  is  compactly 
wrapped  in  6  or  8  grams  of  lead  foil  and  cupeled  in  a  small  cupel 
with  feather  crystals  of  litharge.  The  cupel  should  be  pushed 
back  in  the  muffle  for  the  last  two  or  three  minutes,  to  ensure 
the  removal  of  the  last  of  the  lead.  After  the  play  of  colors 
has  ceased  it  should  be  drawn  toward  the  front  of  the  muffle  and 
then  covered  with  a  very  hot  cupel  to  prevent  sprouting.  It  is 
then  removed  gradually  from  the  muffle  and  when  it  is  cool  the 
bead  is  cleaned,  weighed  and  parted  in  the  ordinary  manner. 
The  gold  will  require  more  than  the  ordinary  amount  of  washing, 
on  account  of  the  large  quantity  of  silver  present. 


THE  ASSAY  OF  BULLION 


227 


If  the  cupeling  has  been  properly  conducted  it  will  be  fair 
to  assume  a  loss  of  1  per  cent  of  silver  in  determining  the  approxi- 
mate silver.  The  weight  of  gold  may  be  taken  as  approximately 
correct.  The  sum  of  the  weights  of  approximate  gold  and  silver 
is  subtracted  from  the  weight  of  bullion  taken  to  obtain  the  amount 
of  base  metal.  This  will  usually  be  copper,  but  the  assayer 
should  be  able  to  determine  what  it  is  from  the  appearance-  of  the 
bullion  and  the  cupel. 

Final  Assay.  —  Three  portions  of  approximately  500  milligrams 
are  weighed  accurately  and  wrapped  in  the  proper  amount  of 
lead  foil  as  shown  by  the  following  table  in  which  the  impurity 
is  assumed  to  be  copper. 

TABLE  XXVI. 
LEAD  RATIO  IN  CUPELLATION. 


Fineness  of  Au.  +  Ag. 

Wt.  of  lead 

Fineness  of  Au.  +  Ag. 

Wt.  of  lead 

950 

5  grams 

750 

11  grams 

900 

7 

700 

12 

850 

8 

650 

13 

800 

10 

600 

15 

Two  checks  are  made  up  with  C.  P.  silver  and  proof  gold,  equal 
to  the  approximate  silver  and  gold  found  by  the  preliminary 
assay,  and  the  necessary  amount  of  copper  or  other  base  metal. 
These  are  wrapped  up  in  the  same  amount  of  sheet  lead  as  was 
used  for  the  bullion.  The  lead  for  these  assays  is  best  cut  into 
equal-sized  rectangles  with  proportions  approximately  1J  inches 
by  2£  inches,  and  twisted  into  the  shape  of  little  cornucopias 
with  the  bottoms  folded  up.  The  bullion  and  metals  going  to 
make  up  the  check  are  transferred  to  these  directly  from  the 
scale-pans,  and  are  then  folded  over  and  made  into  compact 
bundles. 

The  cupels  are  placed  in  a  row  across  the  muffle,  and  when 
they  are  hot,  the  buttons  are  dropped  quickly  into  them  with 
the  checks  in  the  second  and  fourth  cupels.  They  should  be 
cupeled  at  a  low  temperature  so  that  plentiful  crystals  of  litharge 
are  obtained  all  around  the  buttons,  but  toward  the  end  the 


228  A   TEXTBOOK  OF  FIRE  ASSAYING 

temperature  should  be  increased  to  make  sure  of  driving  off  the 
last  of  the  lead. 

The  beads  are  cleaned,  weighed  and  parted,  and  the  gold  is 
weighed.  The  per  cent  loss  of  gold  and  silver  is  determined  and 
a  corresponding  correction  made  to  the  weights  of  gold  and  silver 
found.  From  these  figures  the  fineness  in  both  gold  and  silver  is 
determined.  The  gold  should  check  within  0.1  part  and  the  sil- 
ver within  0.5  parts. 

Notes:  1.  When  the  dore  contains  antimony  the  samples  are  weighed 
into  2.5-inch  scorifiers  with  30  grams  of  test  lead.  Proofs  are  made  up  ac- 
cording to  the  preliminary  assay.  All  are  scorified  in  the  same  muffle  at  the 
same  time.  Should  the  weight  of  these  lead  buttons  vary  over  a  gram,  they 
are  made  up  to  the  same  weight  with  lead  foil  before  cupeling.  The  assay 
is  carried  on  from  this  point  as  if  no  impurities  had  been  present. 

2.  When    the    dore    contains    bismuth,    selenium    or    tellurium,    three 
one-half  gram  portions  are  weighed  out  into  2^ -inch  scorifiers  with  forty 
grams  of  test  lead  and  scorified,  and  the  lead  buttons  are  flattened  out  into 
sheets  about  3  inches  square.     These  sheets  of  lead  are  dissolved  in  about 
200  c.c.,  of  dilute  HNOa  (1-3)  and  the  solutions  are  boiled  to  expel  all  red 
fumes.     They  are  then  diluted  to  400  c.c.,  filtered  through  triple-folded  15 
cm.  filters,  and  the  precipitate  is  washed  once.     To  the  filtrate  is  added  suf- 
ficient NaCl  solution  to  precipitate  all  the  silver.     The  solutions  are  heated 
to  boiling  and  allowed  to  stand  over  night.     The  silver  chloride  is  filtered  off 
through  15  cm.  filters  and  the  precipitate  is  washed  only  once.     The  two  filter 
papers  are  placed  in  a  2^-inch  lead-lined  scorifier,  dried  and  burned  in  an  oven, 
then  covered  with  30  grams  of  test  lead  and  scorified.     When  the  scorifiers 
have  entirely  closed  over,  the  muffle  door  is  closed  and  the  heat  raised.    When 
hot,  the  fusions  are  poured  and  the  lead  buttons  treated  exactly  as  those 
from  bullion  containing  antimony. 

3.  If  the  silver  fineness  of  the  dore  is  not  three  or  more  times  greater 
than  the  gold  fineness,  another  set  of  assays  must  be  run  with  the  addition  of 
sufficient  proof  silver  to  allow  for  parting. 

Instead  of  attempting  to  prevent  sprouting  by  covering  with  a 
hot  cupel,  the  student  may  try  the  following  little-known  method, 
first  described  by  Aaron.*  After  brightening,  the  cupel  is  drawn 
to  the  front  of  the  muffle  and  gently  tapped  on  one  side  with  the 
tongs.  At  the  instant  when  the  bead  ceases  to  vibrate  in  response 
to  the  taps,  by  which  is  indicated  the  beginning  of  solidification, 
it  is  pushed  back  into  the  hottest  part  of  the  muffle  and  left  for 
about  a  minute.  After  this  it  may  be  entirely  withdrawn  and  will 
not  sprout,  being  solid  all  through,  as  shown  by  a  "  dimple  "  in 
its  surface,  caused  by  contraction. 

*  Assaying  Gold  and  Silver  Ores,  p.  67. 


THE  ASSAY  OF  BULLION  229 

On  being  drawn  to  the  front  of  the  muffle,  the  cupel  is  cooled, 
and  as  the  bead  begins  to  solidify  it  is  pushed  back  where  the  heat 
thrown  down  on  it  prevents  the  surface  from  solidifying,  or  melts 
it  again.  The  partially  cooled  cupel,  absorbing  the  heat,  causes 
the  bead  to  solidify  from  below  and  thus  the  gas  is  allowed  to 
escape  quietly. 

UNITED   STATES  MINT  ASSAY  OF  GOLD   BULLION. 

Preliminary  Assay.  Assay  for  Bases.  —  To  determine  the  ap- 
proximate composition  of  the  bullion  a  preliminary  assay  is  made. 
A  sample  of  1000  milliemes  (500  mg.)  is  weighed  out,  wrapped  in 
five  grams  of  lead  foil,  and  cupeled.  The  weight  of  the  bullion 
taken,  less  the  weight  of  the  bead  obtained,  gives  the  base  met- 
als. 

The  bead  now  consists  of  gold  and  silver,  the  approximate 
relative  proportions  of  which  must  be  determined.  This  may  be 
done  by  adding  silver,  cupeling  and  parting,  or  by  touchstone. 
This  latter  method  is  used  at  the  Government  Assay  Offices 
and  Mints.  The  touchstone  method  consists  in  rubbing  the 
sample  on  a  piece  of  black  jasper  and  comparing  the  mark  with 
marks  made  by  alloy  slips,  "  needles,"  of  known  composition. 
The  needles  range  from  500  to  1000  fine  and  are  20  points  apart. 
This  gives  the  fineness  within  2  per  cent,  which  is  close  enough 
to  show  how  much  silver  to  add  in  order  to  inquart  the  main 
assay  and  to  make  up  the  check  or  proof  center. 

Final  Assay.  —  The  final  assay  is  usually  made  by  two  assayers, 
each  working  on  one  of  the  chip  or  drill  samples.  In  the  case  of 
a  small  bar,  each  makes  one  assay,  while  in  the  case  of  a  large 
bar  each  assayer  makes  two  or  more  assays.  The  balance  used 
for  the  assay  is  usually  adjusted  so  that  a  deviation  of  the 
needle  of  1  division  on  the  ivory  scale  amounts  to  some  simple 
fraction  of  the  weights  used.  Thus,  at  one  assay  office  a  deviation 
of  the  swing  of  1  division  on  the  ivory  scale  amounts  to  0. 1  mg.  = 
0.2  milliemes.  With  this  adjustment  it  is  not  necessary  to  make 
so  many  trials  with  the  rider  to  get  the  final  weight,  nor  is  it 
necessary  to  weigh  out  exactly  an  even  half  gram  of  bullion  for 
the  assay.  Instead  we  weigh  out  1000  ±  3  divisions  on  the 
ivory  scale,  record  the  difference,  and  make  a  corresponding  cor- 
rection when  the  gold  cornet  is  weighed. 


230  A   TEXTBOOK  OF  FIRE  ASSAYING 

As  stated  above  the  weight  of  bullion  taken  for  each  assay  is 
1000  milliemes.  To  this  is  added  sufficient  silver  to  make  the 
ratio  of  silver  to  gold  2  to  1,  and  the  whole  is  wrapped  up  in  5 
or  6  grams  of  lead  foil.  The  lead  foil  pieces  are  all  cut  to  exact 
size,  about  1J  inches  by  2J  inches,  and  rolled  up  into  the  shape 
of  a  cornucopia  with  the  bottom  pinched  in.  The  bullion  is 
poured  directly  into  these  from  the  scale-pan.  The  silver  is 
added  in  the  form  of  discs  made,  for  convenience,  in  four  or  five 
different  sizes.  These  discs  are  punched  out  of  sheets  carefully 
rolled  to  gage,  so  that  the  punchings  will  weigh  exactly  even  tens 
and  hundreds  in  the  gold  weight  system.  If  the  bullion  contains 
no  copper  it  is  advisable  to  add  about  30  milli&mes.  This  copper 
may  be  alloyed  with  the  silver  used  for  parting. 

One  or  more  proofs  of  pure  gold  weighing  usually  900  milliemes 
(0.450  gram)  are  also  weighed  and  made  up  to  the  2  to  1  ratio, 
and  copper  is  added  to  approximate  that  in  the  bullion.  These 
are  wrapped  in  the  same  quantity  of  lead  foil  as  the  bullion,  and 
one  or  more  are  run  in  each  row  of  cupels  in  the  muffle.  The 
lead  packets  are  pressed  into  spherical  shape  with  pliers  specially 
designed  for  the  purpose. 

The  lead  packets  are  put  in  order  as  prepared  in  the  numbered 
compartments  of  a  wooden  tray  and  taken  to  the  furnace  room 
where  they  are  cupeled  in  a  rather  hot  muffle.  The  cupels  are 
surrounded  by  a  row  of  extra  cupels  so  that  the  temperature  may 
be  kept  as  uniform  as  possible  for  all  the  assays.  The  cupels 
are  withdrawn  while  the  beads  are  still  fluid.  With  a  2  to  1 
ratio  of  silver  to  gold,  and  with  copper  present,  there  is  no  danger 
of  sprouting. 

The  beads  are  removed  from  the  cupels  by  means  of  pliers  and 
carefully  cleaned  from  all  adhering  bone-ash.  They  are  then 
placed  on  a  special  anvil  and  flattened  by  a  middle  blow  and  two 
end  blows  with  a  heavy  polished  hammer.  They  are  then  an- 
nealed at  a  dull  red  heat  and  passed  twice  through  the  rolls  which 
are  adjusted  each  time,  so  that  after  the  second  passage  they  are 
about  2|  inches  long  by  J  inch  wide,  and  about  as  thick  as  an  or- 
dinary visiting  card.  It  is  important  that  the  fillets  be  all  of  the 
same  size  and  thickness  and  that  they  have  smooth  edges.  They 
are  then  reannealed  and  each  one  is  numbered  on  one  end  with 
small  steel  dies  to  correspond  with  the  number  of  the  assay, 
after  which  they  are  rolled  up  into  "  cornets  "  or  spirals  between 


THE  ASSAY  OF  BULLION 


231 


the  finger  and  thumb,  with  the  number  outside.  It  is  important 
that  an  even  space  be  left  between  all  turns  of  the  spiral,  in  order 
that  the  acid  shall  have  easy  access  to  all  parts  of  the  gold. 

The  cornets  are  parted  in  platinum  thimbles,  which  are  sup- 
ported in  a  platinum  basket,  and  the  whole  is  placed  in  a  platinum 
vessel  containing  boiling  nitric  acid  of  32°  B.  (Sp.  Gr.  1.28). 
They  are  boiled  for  ten  minutes  and  then  transferred  to  another 
vessel  containing  acid  of  the  same  strength  and  boiled  ten  min- 


FIG.  51.  —  Stages  in  preparing  bead  for  parting  in  gold  bullion  assay,  (a)  bead 
(6)  after  flattening  (c)  fillet  (d)  cornet  before  parting  (e)  cornet  after 
parting  and  annealing. 


utes  longer.  The  basket,  with  its  contents  is  then  washed  by 
dipping  it,  vertically  in  and  out,  in  three  changes  of  distilled 
water.  It  is  now  drained,  dried,  and  annealed,  usually  in  the 
muffle. 

The  various  stages  in  the  conversion  of  the  bead  to  the  parted 
cornet  are  shown  in  Fig.  51. 

When  cold,  the  cornets  are  ready  to  be  weighed.  The  gold 
should  be  entirely  in  one  piece,  and  the  original  numbers  easily 
discernible  on  the  parted  cornets.  The  proofs  are  weighed  first 
and  the  corrections  applied  to  the  weight  of  the  other  cornets. 
The  proofs  always  show  a  slight  gain  in  weight.  The  correction 


232  A    TEXTBOOK  OF  FIRE  ASSAYING 

thus  determined  is  termed  the  "  surcharge,"  and  is  really  the 
algebraic  sum  of  all  the  gains  and  losses. 

When  more  than  fourteen  cornets  are  parted  at  one  time  the 
lot  is  given  a  preliminary  three  minute  treatment  in  an  extra 
lot  of  acid,  followed  by  the  two  regular  ten  minute  boilings. 

The  .purpose  of  the  copper  which  is  added  to  the  assays  is  to 
render  the  button  tough  and  permit  of  its  being  rolled  out  into 
a  smooth-edged  fillet.  Without  the  copper,  the  fillet  is  apt  to 
crack  in  rolling,  or  to  come  through  with  a  ragged  edge  which 
might  give  rise  to  a  loss  in  parting.  The  action  of  copper  in 
this  case  is  probably  due  to  its  effect  in  aiding  in  the  removal 
of  the  last  of  the  lead  in  cupeling.*  The  time  required  for  cupella- 
tion  is  approximately  twelve  minutes. 

*  Rose.  Trans.  Inst.  Min.  Met.,  14,  p.  545. 


CHAPTER  XL 
THE  ASSAY  OF  SOLUTIONS. 

A  large  variety  of  methods  for  the  assay  of  gold-  and  silver- 
bearing  solutions  have  been  published  in  the  technical  press, 
and  quite  a  number  of  these  have  been  adopted  by  assayers. 
These  methods  may  be  classified  as  follows: 

1.  Methods  involving  evaporation  in  lead  trays  with  subse- 
quent cupellation,  or  scorification  and  cupellation,  of  the  tray 
and  contents. 

2.  Methods   involving   evaporation   with   litharge   and   other 
fluxes,  followed  by  a  crucible  fusion  and  cupellation. 

3.  Methods  in  which  the  precious  metals  are  precipitated  and 
either  cupeled  directly,  or  first  fused  or  scorified  and  then  cupeled. 

4.  Electrolytic  methods  in  which  the  precious  metals  are  de- 
posited directly  on  cathodes  of  lead  foil,  which  are  later  wrapped 
up  with  the  deposit  and  cupeled. 

5.  Colorimetric  methods  (for  gold  only)  all  of  which  depend 
upon  obtaining  the   "  purple  of  Cassius"   color  which  may  be 
compared  with  proper  standards. 

Evaporation  in  Lead  Tray.  —  This  method  is  a  good  one  for 
rich,  neutral  solutions  containing  only  salts  of  the  precious  metals. 
A  tray  of  suitable  size  is  made  by  turning  up  the  edges  of  a  piece 
of  lead  foil.  If  many  of  these  assays  are  to  be  made  it  is  well 
to  have  a  wooden  block  as  a  form  on  which  the  trays  'may  be 
shaped.  A  tray  2  by  2  inches  and  f  inch  deep  is  about  right 
to  hold  1  assay-ton  of  solution. 

Having  made  a  tray  which  will  not  leak,  the  assayer  adds  the 
solution  and  carefully  evaporates  it  to  prevent  spattering.  The 
tray  is  then  folded  into  a  compact  mass  and  dropped  into  a  hot 
cupel. 

Among  the  disadvantages  of  the  method  are  the  following: 
It  does  not  permit  of  the  use  of  a  large  quantity  of  solution, 
and  therefore  is  suited  only  to  rich  solutions.  If  the  solutions 
are  acid  they  will  corrode  the  tray,  and  if  they  contain  salts 
other  than  those  of  gold  and  silver  these  will  interfere  with  cupel- 

233 


234  A    TEXTBOOK  OF  FIRE  ASSAYING 

lation.  As  both  AuCls  and  KAu(CN)2  are  volatile  at  moderate 
temperatures,  many  assayers  do  not  consider  the  method  a  re- 
liable one  for  solutions  of  these  salts  on  account  of  the  possibility 
of  loss  of  gold. 

Evaporation  with  Litharge.  —  (First  Method).  A  measured 
quantity  of  the  solution  is  placed  in  a  porcelain  evaporating 
dish  and  from  30  to  60  grams  of  litharge  is  sprinkled  over  the  sur- 
face. The  mixture  is  allowed  to  evaporate  at  a  gentle  heat  to 
prevent  both  spitting  and  baking  of  the  residue.  When  dry  the 
residue  is  scraped  out,  mixed  with  suitable  fluxes,  transferred  to 
a  crucible  and  fused  in  the  ordinary  manner.  The  last  portions 
remaining  on  the  dish  may  be  removed  by  means  of  a  small 
piece  of  slightly  moistened  filter  paper  which  is  afterwards  added 
to  the  charge. 

Some  assayers  add  a  little  fine  silica  and  charcoal  with  the 
litharge.  The  soluble  constituents  of  a  crucible  charge,  soda  and 
borax,  should  not  be  added  to  the  solution  as  they  form  a  hard 
cake  which  is  difficult  to  remove  from  the  dish.  The  most  im- 
portant point  in  the  process  is  the  proper  control  of  the  tempera- 
ture. If  this  is  right,  there  will  be  no  spattering  and  the  dry 
residue  will  come  away  from  the  dish  practically  clean,  after 
it  has  been  pried  up  with  the  point  of  a  spatula. 

Evaporation  with  Litharge.  —  (Second  Method) .  A  measured 
amount  of  solution  is  evaporated  to  a  small  volume  in  a  porcelain 
or  enameled  iron  dish,  without  the  addition  of  any  reagents, 
and  the  concentrated  solution  is  then  transferred  to  a  small 
dish  of  very  thin  glass,  known  as  a  Hoffmeister's  dish.  The 
solution  is  evaporated  to  dryness  either  with  or  without  litharge, 
and  the  dish  and  contents  broken  up  directly  into  a  crucible 
containing  the  usual  fluxes.  The  assay  is  finished  in  the  usual 
manner.  The  advantage  of  this  method  lies  in  the  fact  that 
there  is  no  chance  of  losing  any  of  the  residue  by  not  properly 
cleaning  the  dish,  as  the  dish  and  all  are  fused. 

The  evaporation  method,  while  somewhat  long,  is  the  most 
reliable  and  accurate  one  known,  and  is  the  standard  with  which 
all  other  methods  are  compared.  If  arrangements  are  made  for 
allowing  the  evaporation  to  run  over  night,  the  samples  taken 
one  night  may  be  assayed  and  reported  early  next  morning.  The 
method  is  adapted  to  the  treatment  of  solutions  in  any  quantity 
and  of  almost  any  character.  If  the  solution  contains  much  sul- 


THE  ASSAY  OF  SOLUTIONS  235 

phuric  acid,  the  litharge  may  be  converted  into  lead  sulphate, 
which  is  not  suited  either  to  act  as  a  flux  or  to  provide  lead  for 
a  collecting  agent.  A  fusion  made  on  such  a  substance  with  a 
carbonaceous  reducing  agent,  will  give  either  no  button  at  all, 
or  a  button  of  matte.  The  reaction  between  lead  sulphate  and 
carbon  is  as  follows : 

PbS04  +  2C  =  PbS  +  2C02. 

If  the  solution  is  one  of  AuCl3,  a  little  charcoal  should  be  added 
during  the  evaporation,  to  ensure  the  reduction  and  precipitation 
of  the  gold,  as  in  this  way  we  avoid  the  danger  of  loss  of  gold  by 
volatilization  as  the  chloride.  The  gold,  being  precipitated  on 
the  charcoal,  is  in  the  best  possible  position  to  be  alloyed  with 
the  lead  which  will  be  reduced  by  the  carbon. 

Precipitation  by  Zinc  and  Lead  Acetate.  The  Chiddey  Method. 
—  (For  Cyanide  Solutions).  This  method,  which  was  first  de- 
scribed by  Alfred  Chiddey*  is  suitable  for  both  gold  and  silver  and 
is  used  almost  exclusively  in  this  country  for  the  assay  of  cyanide 
solutions.  It  works  equally  well  on  strong  or  weak,  foul  or  pure 
solutions,  and  almost  any  quantity  may  be  taken.  Many  changes 
of  detail  have  been  suggested  and  innumerable  modifications 
of  the  original  process  have  been  described  in  the  technical  press. 
The  following  method  has  been  found  satisfactory: 

Take  from  1  to  20  assay-tons  of  solution  in  a  beaker  or  evaporat- 
ing dish,  and  heat.  Add  10  or  20  c.c.  of  a  10  per  cent  solution 
of  lead  acetate  containing  40  c.c.  of  acetic  acid  per  liter.  Then 
add  1  or  2  grams  of  fine  zinc  shavings  rolled  lightly  into  a  ball. 
The  gold,  silver  and  lead  will  immediately  commence  to  precipi- 
tate on  the  zinc.  At  first  the  solution  may  become  cloudy  but 
will  soon  clear  as  more  of  the  lead  is  precipitated.  Heat,  but  not 
to  boiling,  until  the^lead  is  well  precipitated.  This  usually  takes 
about  twenty  or  twenty-five  minutes.  Then  add  slowly  (about 
5  c.c.  at  a  time),  20  c.c.  hydrochloric  acid  (1.12  sp.  gr.),  to  dissolve 
the  excess  zinc.  Continue  heating  until  effervescence  stops.  It 
is  often  found  that  action  ceases  while  there  is  still  some  undis- 
solved  zinc  remaining.  This  is  entirely  covered  and  thus  pro- 
tected from  the  acid  by  the  spongy  lead.  To  be  sure  that  all 
the  zinc  is  dissolved,  feel  of  the  sponge  with  a  stirring  rod  and 
drop  a  little  hydrochloric  acid  from  a  pipette  directly  on  it. 
*  Eng.  and  Min.  Jour.,  75,  p.  473,  (1903). 


236  A   TEXTBOOK  OF  FIRE  ASSAYING 

As  soon  as  the  zinc  is  dissolved  decant  off  the  solution  and  wash 
the  sponge  two  or  three  times  with  tap  water.  Next,  moisten 
the  fingers  and  press  the  sponge,  which  should  be  all  in  one  piece, 
into  a  compact  mass.  Dry  by  squeezing  between  pieces  of  soft 
filter  paper  or  by  placing  on  a  piece  of  lead  foil  and  rolling  with 
a  piece  of  large  glass  tubing.  Finally  roll  into  a  ball  with  lead 
foil,  puncture  to  allow  for  escape  of  steam,  add  silver  for  parting, 
and  place  in  a  hot  cupel. 

As  soon  as  the  zinc  is  dissolved  the  assay  should  be  removed 
from  the  heat,  and  the  sponge  removed.  If  this  is  not  done  the 
lead  will  start  to  dissolve  and  the  sponge  will  soon  break  up. 
Washing  by  decantation  and  manipulation  with  the  fingers  may 
appear  crude,  but  after  a  little  practice  the  operator  becomes  so 
proficient  that  there  is  practically  no  chance  of  losing  any  of  the 
lead. 

If  any  considerable  amount  of  water  is  left  the  assay  will  spit 
in  the  cupel.  To  avoid  this  danger  some  assayers  dry  the  assays 
on  the  steam  table  before  cupeling.  Any  zinc  left  will  also  prob- 
ably cause  spitting.  Chiddey  recommends  placing  a  piece  of  dry 
pine  wood  in  the  mouth  of  the  muffle  immediately  after  charging 
the  cupels,  probably  with  the  idea  that  this  aids  in  preventing 
spitting  when  some  zinc  has  been  left  undissolved.  When  work- 
ing with  small  quantities  of  solutions  it  is  best  to  add  water  oc- 
casionally to  maintain  a  volume  of  at  least  100-150  c.c.  The 
secret  of  keeping  the  lead  from  breaking  up  is  not  to  allow  the 
solution  to  come  to  a  boil  at  any  stage  of  the  procedure. 

Zinc  dust  is  used  by  many  chemists  in  place  of  zinc  shavings, 
a  small  amount  being  added  on  the  end  of  a  spatula.  Many 
chemists  agree  that  half  a  gram  is  sufficient. 

William  H.  Barton*  suggests  the  addition  of  a  small  piece 
of  aluminum  foil  dropped  into  the  solution  after  the  hydrochloric 
acid  is  added,  to  prevent  the  dissolving  of  the  lead  and  the  con- 
sequent breaking  up  of  the  sponge  by  the  hydrochloric  acid  after 
the  zinc  is  all  dissolved. 

T.  P.  Holtf  recommends  the  substitution  of  a  square  of  alu- 
minum foil  for  the  zinc.  The  lead  sponge  is  removed  from  the 
aluminum  with  a  rubber-tipped  stirring  rod.  Care  must.be  taken 
to  use  a  sufficiently  thick  sheet  of  aluminum  (1/16  inch  does 

*  Western  Chemist  and  Metallurgist,  4,  p.  67,  (1908). 
f  Min.  and  Sci.  Press,  100,  p.  863,  (1910). 


THE  ASSAY  OF  SOLUTIONS  237 

well),  to  prevent  small  pieces  becoming  detached.  These  would 
remain  with  the  lead  sponge  and  might  cause  the  cupels  to  spit. 

Precipitation  as  Sulphide.*  —  Acidify  5  or  10  assay-tons  of 
solution  with  HC1  and  heat  to  boiling.  While  it  is  boiling  add  a 
solution  containing  2  grams  of  lead  acetate  and  pass  in  a  current 
of  hydrogen  sulphide  until  all  the  lead  is  precipitated.  Allow 
to  cool  somewhat,  still  passing  in  H2S,  then  filter  and  dry.  Col- 
lect the  gold  and  silver  with  lead,  either  by  a  crucible  fusion  or  a 
scorification  assay.  The  method  is  said  to  be  quick,  accurate 
and  economical. 

Precipitation  by  Cement  Copper,  f  —  To  8  assay-tons  of  the 
solution  add  a  few  cubic  centimeters  of  sulphuric  acid,  and  1 
gram  of  finely  divided  cement  copper.  Heat  to  boiling  and  boil 
ten  minutes.  Filter  through  a  strong  7-inch  paper  and  place  on 
the  drained  filter  one-third  of  a  crucible  charge  of  mixed  flux. 
Place  the  filter  in  a  crucible  containing  another  third  of  a  charge 
of  flux,  and  cover  with  the  final  third.  Fuse  and  cupel  as  usual. 
The  filter  itself  furnishes  the  reducing  agent  for  the  assay.  If 
cement  copper  is  not  available,  a  solution  of  copper  sulphate 
may  be  added,  together  with  a  small  piece  of  aluminum  foil. 
Boil  until  all  the  copper  is  precipitated  and  add  the  remaining 
aluminum  foil  to  the  fusion.  This  modification  takes  more  time 
than  the  first. 

Precipitation  by  Silver  Nitrate.J  —  (For  Gold  in  Cyanide  Solu- 
tions). Add  an  excess  of  silver  nitrate  solution  which  will  cause 
the  gold  and  silver  to  precipitate  as  an  auric-argentic-cyanide. 
Allow  the  precipitate  to  settle,  filter  through  a  thin  paper,  and 
wash  several  times.  Dry  the  filter  and  either  scorify  with  test 
lead  or  fuse  in  a  crucible  with  litharge  and  the  regular  fluxes. 
The  method  gives  fairly  good  results  with  solutions  not  too  low  in 
gold.  With  solutions  very  low  in  gold  the  precipitation  of  the 
gold  is  not  perfect. 

Precipitation  by  a  Copper  Salt.§  —  (For  Cyanide  Solutions  Only). 
Add  to  1  liter  of  solution  in  a  2-liter  flask  25  c.c.  of  a  10  per  cent 
solution  of  copper  sulphate,  then  add  5  to  7  c.c.  of  concentrated 

*  Henry  Watson,  Eng.  and  Min.  Jour.,  66,  p.  753,  (1898). 
f  Albert  Arents,  Trans.  A.I.M.E.,  34,  p.  184. 

J  Andrew  F.  Cross,  Jour.  Chem.  Met.  and  Min.  Soc.  of  South  Africa, 
1,  p.  28,  and  3,  p.  1. 

§  A.  Whitby,  Jour.  Chem.  Met.  and  Min.  Soc.  of  South  Africa,  3,  p.  6. 


238  A   TEXTBOOK  OF  FIRE  ASSAYING 

hydrochloric  acid  and  lastly  10  to  20  c.c.  of  a  10  per  cent  solu- 
tion of  sodium  sulphite.  Shake  vigorously  for  at  least  two  min- 
utes, then  filter,  dry,  and  fuse  the  filter  and  precipitate  in  the 
usual  way.  With  weak  solutions  it  is  best  to  bring  up  the 
strength  by  the  addition  of  cyanide  before  adding  the  copper 
salt.  The  gold  and  silver  are  carried  down  by  the  precipitate 
of  cuprous  cyanide  formed.  Assays  may  be  completed  in  three 
hours,  and  the  results  are  said  to  be  good  on  both  low-  and  high- 
grade  solutions. 

The  Electrolytic  Assay  of  Cyanide  Solutions.  —  The  following 
method  is  abstracted  from  the  Journal  of  the  Chemical,  Metal- 
lurgical and  Mining  Society  of  South  Africa*  in  which  is  described 
the  method  and  installation  used  at  the  Kleinfontein  Group 
Central  Administration  Assay  Offices. 

Ten-assay-ton  samples  of  the  solution  to  be  assayed  are  placed 
in  No.  3  beakers,  which  are  held  in  a  frame,  and  electrolyzed 
with  a  current  of  0.1  ampere.  The  anodes  used  consist  of  ordin- 
ary T\-inch  arc  lamp  carbons  which  are  held  in  position  in  the 
center  of  each  beaker  by  suitable  clamps.  They  are  arranged 
so  that  they  may  be  lifted  out  of  the  solution  when  no  current  is 
passing.  The  cathodes  are  made  from  strips  of  ordinary  assay 
lead  foil  2J  by  9  inches,  with  the  lower  edge  coarsely  serrated 
to  allow  for  circulation  of  the  solution.  To  connect  with  the 
battery  a  J-inch  strip  is  almost  severed  from  one  end  of  the  foil, 
and  turned  upward  to  make  a  terminal.  The  two  ends  of  the 
lead  are  brought  together  and  connected  by  folding  the  edges, 
making  a  cylinder  about  3  inches  in  diameter. 

The  time  required  for  the  complete  deposition  of  the  gold  is 
four  hours,  after  which  the  carbons  are  removed,  the  lead  cathodes 
disconnected  and  dried  on  a  hot-plate.  When  dry,  they  are 
folded  into  a  compact  mass  and  cupeled. 

With  weak  solutions  a  small  quantity  of  cyanide  should  be 
added  in  order  to  decrease  the  resistance  and  thus  accelerate 
the  deposition  of  the  precious  metals.  The  author  reports  no 
difficulty  in  obtaining  a  complete  and  adherent  deposit  of  the 
gold,  which  separates  as  a  bright  yellow  deposit. 

This,  of  course,  was  the  only  metal  worked  for  on  the  Rand, 
but  there  seems  to  be  no  reason  why  silver  as  well  as  gold  cannot 
be  determined  by  this  method. 

*.Vol.  12,  p.  90,  C.  Crichton. 


THE  ASSAY  OF  SOLUTIONS  239 

The  principal  advantage  of  the  method  lies  in  the  small  amount 
of  actual  personal  attention  required.  The  method  works  as 
well  for  a  20  assay-ton  sample  as  for  one  of  10  assay-tons.  The 
time  required  for  the  deposition  of  the  gold  is  somewhat  longer 
than  for  some  of  the  precipitation  methods  and  this  appears  to 
be  the  principal  disadvantage  of  the  process. 

Colorimetric  Methods.  —  (For  Gold  only).  Several  attempts 
have  been  made  to  adapt  the  "  Purple  of  Cassius  "  test  to  the 
estimation  of  gold  in  chloride  and  cyanide  solutions.  So  far  as 
the  author  is  aware,  none  of  the  methods  have  beeen  adopted  as 
practical  assay  laboratory  methods  in  this  country.  They  were 
used  for  a  time  in  one  or  two  South  African  plants,  but  have 
never  come  into  great  favor.  The  two  most  promising  methods 
were  described  by  Henry  R.  Cassel  (Eng.  and  Min.  Jour.  76 
p.  661)  and  James  Moir  (Proc.  Chem.  Met.  and  Min.  Soc.  of 
South  Africa,  4,  p.  298),  and  to  those  original  articles  the  inter- 
ested reader  is  referred. 


CHAPTER  XII. 
THE  LEAD   ASSAY. 

The  fire  assay  for  lead  consists  of  a  reducing  fusion  with  iron,  • 
fluxes,  and  some  carbonaceous  reducing  agent,  and  is  conducted 
much  as  is  the  iron-nail  assay  for  gold  and  silver  ores,  except, 
of  course,  that  no  litharge  or  other  lead-bearing  flux  is  added. 
The  object  of  the  fusion  is  to  reduce  and  collect  all  of  the  lead 
in  a  button  free  from  other  elements. 

Lead  Ores.  —  Lead  ores  are  classified  by  metallurgists  as 
oxidized  or  sulphide  ores,  also  as  pure  or  impure  ores.  The 
oxidized  ores  contain  the  lead  principally  in  the  form  of  carbon- 
ate, occasionally  as  sulphate  and  rarely  as  oxide  or  in  combina- 
tion with  phosphorous,  molybdenum,  vanadium,  chromium,  etc. 
The  corresponding  lead  minerals  are  cerussite,  PbCO3  (77.6  per 
cent  Pb),  anglesite  PbSO4  (68.3  per  cent  Pb),  minium  Pb3O< 
(90.6  per  cent  Pb),  pyromorphite  Pb5Cl  (PO4)3  (75.6  per  cent 
Pb),  vanadinite  3Pb3(VO4)2  PbCl2  (72.4  per  cent  Pb)  and  wulfen- 
ite  PbMoO4  (56.5  per  cent  Pb).  The  most  important  sulphide 
lead  minerals  are  galena  PbS  (86.6  per  cent  Pb)  jamesonite 
Pt^SbaSo  (50.8  per  cent  Pb)  and  bournonite  PbCuSbS3  (42.5  per 
cent  Pb).  The  principal  associated  minerals  are  argentite,  py- 
rite,  chalcopyrite,  sphalerite,  stibnite,  quartz,  calcite  and  dol- 
omite, as  well  as  the  oxidation  compounds  of  the  above  sul- 
phides. Impure  ores,  from  the  assayer's  point  of  view,  are  those 
containing  more  or  less  arsenic,  antimony,  bismuth,  copper, 
zinc,  and  other  rarer  metals  which  interfere  with  the  lead  assay. 

Besides  ores,  the  assayer  may  have  brought  to  him  various 
furnace  products  such  as  litharge,  slag,  matte,  flue  dust  and  cu- 
pel bottom. 

The  fire  assay  for  lead  is  not  as  accurate  as  a  carefully  made 
wet  determination,  but  it  is  so  simple,  inexpensive  and  rapid 
that  for  a  long  time  it  served  to  govern  the  purchase  and  sale 
of  all  lead  ores.  Today  it  is  still  largely  used  by  the  smelters 
and  others  for  the  assay  of  pure  ores,  although  for  ores  contain- 

240 


THE  LEAD  ASSAY  241 

ing  such  base  metal  impurities  as  antimony,  copper,  zinc,  etc., 
the  wet  method  is  usually  preferred.  The  results  of  the  fire-assay 
may  be  either  lower  or  higher  than  the  actual  lead  content,  de- 
pending on  the  nature  and  quantity  of  the  other  minerals  present 
in  the  ore. 

Pure  ores  give  low  results  owing  to  losses  of  lead  by  volatiliza- 
tion and  slagging.  Both  the  sulphide  and  the  oxide  of  lead  are 
volatile  at  moderate  temperatures  and  for  this  reason  great  care 
must  be  taken  to  maintain  the  lowest  temperature  consistent 
with  a  proper  decomposition  of  these  minerals,  during  the  early 
part  of  the  assay.  Lead  oxide  begins  to  volatilize  at  about 
800°  C.,  and  the  loss  due  to  this  cause  is  rapid  at  1000°.  Lead 
sulphide  is  more  easily  volatilized  than  the  oxide.  In  a  neutral 
or  reducing  atmosphere  Doeltz*  found  that  at  860°  C.,  it  lost 
18  per  cent  in  an  hour,  while  at  950°  it  vaporized  at  the  rate  of 
45  per  cent  per  hour.  Lead  compounds,  particularly  the  oxide, 
also  tend  to  pass  into  the  slag  and  this  tendency  is  increased  by 
the  presence  of  zinc,  and  to  some  extent  by  arsenic  and  antimony. 

Impure  ores  containing  arsenic,  antimony,  bismuth  and  copper 
usually  give  high  results,  as  these  metals  are  partly  or  wholly 
reduced  and  pass  into  the  lead  button. 

Quantity  of  Ore  and  Reagents  Used.  —  The  amount  of  ore 
used  is  generally  10  grams,  occasionally  5  grams.  With  low- 
grade  ores  20,  25,  or  more  grams  may  be  used.  The  reagents 
used  are  the  alkali  carbonates,  borax-glass,  some  reducing 
agent,  usually  argols  or  flour,  and  occasionally  sulphur.  Iron 
in  some  form  is  always  used.  It  may  be  in  the  form  of 
nails  or  spikes,  or  coiled  wire,  or  the  crucible  itself  may  be  of 
iron,  and  in  this  case  will  be  used  over  and  over  again  until  worn 
out.  A  very  satisfactory  way  of  introducing  iron  is  to  use  a 
rail-  or  boat-spike  2J  or  3  inches  long,  and  about  f  inch  through. 
In  this  assay  it  is  customary  to  use  a  mixture  of  sodium  and  po- 
tassium carbonates,  as  the  mixture  fuses  at  a  lower  temperature 
than  either  one  alone.  The  alkali  carbonates  act  as  fluxes  for  the 
silica,  and  serve  to  give  a  basic  slag  which  is  necessary  in  this 
assay.  Usually  two  or  three  times  as  much  alkaline  carbonate 
as  ore  is  taken.  Borax-glass  acts  as  a  flux  for  the  metallic  oxides, 
for  limestone  and  the  other  alkaline  earths.  From  one-half  to 
twice  as  much  borax-glass  as  ore  is  used.  An  excess  of  reducing 

*  Metallurgie,  3,  p.  441. 


242  A   TEXTBOOK  OF  FIRE  ASSAYING 

agent  is  always  used  to  maintain  the  highly  reducing  character 
of  the  slag  which  is  required.  Sulphur  is  used  when  an  oxidized 
ore  containing  copper  is  being  assayed. 

In  the  lead  assay  it  is  customary  to  use  a  mixed  flux  called  a 
"  lead  flux."  This  may  be  bought  already  prepared  or  may  be 
made  up  in  the  laboratory.  Many  different  formulas  are  given, 
including  the  following: 

1  2  3 

Sodium  carbonate         12  parts  4  parts  6.5  parts 

Potassium  carbonate    15       "  4       "  5.0      " 

Borax-glass  7       "  2.5      " 

Borax  powdered  2       " 

Flour  2       "  1       "  2.5      " 

Nos.  1  and  2  are  found  in  use  in  the  Coeur  d'  Alene  lead  dis- 
trict where  the  fire  assay  for  lead  has  been  brought  to  the  highest 
degree  of  perfection.  No.  1  is  better  for  ores  having  a  basic  gangue, 
No.  2  for  siliceous  ores.  No.  3  is  perhaps  the  best  of  all  for 
general  use. 

About  30  grams  of  flux  are  intimately  mixed  with  10  grams 
of  ore,  one  spike  or  four  or  five  10-penny  nails  are  inserted  and  a 
cover  of  8  or  10  grams  more  of  flux  is  added.  Very  few  assayers 
use  a  cover  of  salt  in  the  lead  assay,  on  account  of  the  danger 
of  the  loss  of  lead  as  chloride. 

The  fusion  should  always  be  made  in  a  muffle  furnace  owing 
to  the  better  control  of  temperature  available.  In  fact,  the 
secret  of  the  successful  fire-assay  for  lead  is  largely  in  the  proper 
manipulation  and  control  of  the  temperature  throughout  the 
process. 

At  first  the  muffle  should  be  just  visibly  red  and  the  crucibles 
should  be  allowed  to  remain  at  this  temperature  for  about  twenty 
minutes.  Then  the  heat  should  be  gradually  raised  until  fusion 
begins,  and  kept  at  this  temperature  for  some  time. 

This  is  necessary  owing  to  the  fact  that  in  the  early  part  of 
the  assay  the  charge  is  in  active  motion  and  particles  of  the 
various  lead  compounds  are  continually  being  brought  to  the 
surface,  where,  if  the  temperature  were  high,  they  would  suffer 
an  appreciable  loss  by  volatilization.  When  the  charge  has 
finished  boiling  and  most  of  the  lead  is  reduced  and  collected  in 
the  bottom  of  the  crucible  there  is  less  danger  of  loss  by  volatili- 


THE  LEAD  ASSAY  243 

zation,  first,  because  lead  itself  is  not  so  readily  volatile  as  are 
some  of  its  compounds,  and  second,  because  it  is  difficult  for  the 
molecules  to  migrate  through  the  heavy  layer  of  reducing  slag 
which  covers  the  lead. 

After  the  boiling  has  entirely  ceased  the  temperature  is  raised 
to  the  highest  heat  of  the  muffle  to  decompose  the  lead  compounds 
which  still  remain  in  the  slag.  These  are  principally  the  silicate 
and  the  double  sulphide  of  lead  and  sodium  or  potassium,  and 
require  a  bright-yellow  heat  for  their  complete  decomposition. 
The  fusion  period  is  finished  when  the  nails  can  be  removed  free 
from  shots  of  lead.  Sulphide  ores  require  a  much  longer  fusion 
than  oxides,  owing  to  the  fact  that  their  decomposition  is  effected 
principally  by  iron,  and  therefore  time  must  be  allowed  for  every 
particle  of  the  charge  to  come  into  contact  with  the  iron.  Oxide 
ores,  on  the  other  hand,  are  decomposed  by  the  carbon  of  the 
charge  and  as  this  is  uniformly  distributed  a  much  shorter 
time  will  suffice.  Sulphide  ores  will  require  from  an  hour  to  an 
hour  and  a  half  of  fusion,  oxide  ores  from  three-quarters  of  an 
hour  to  an  hour. 

Influence  of  Other  Metals  on  Lead  Assay.  SILVER.  —  Practi- 
cally all  of  the  silver  in  an  ore  is  reduced  and  passes  into  the  lead 
button.  If  it  is  present  in  sufficiently  large  quantities  a  correc- 
tion for  it  may  be  made,  i.e.,  291.66  ounces  per  ton  equals  1  percent. 

GOLD.  —  This  metal  is  also  reduced  and  passes  into  the  lead 
button,  but  it  is  usually  present  in  such  small  quantities  that  it 
may  be  disregarded. 

ARSENIC.  —  Arsenic  is  occasionally  found  in  lead  ores,  usually 
in  the  form  of  arsenical  iron  pyrite.  During  the  assay,  part  of  the 
arsenic  is  volatilized  as  metal  or  as  arsenic  sulphide  but  the  larger 
part  remains  in  the  crucible.  Here  it  usually  enters  into  com- 
bination with  the  iron,  forming  speiss.  After  the  contents  of 
the  crucible  has  been  poured,  the  arsenic  will  be  found  as  a  hard 
white  button  on  top  of  the  lead,  from  which  it  may  be  removed 
by  hammering.  Little  if  any  arsenic  enters  the  lead  button. 
Under  certain  conditions,  i.e.,  a  long  fusion  at  a  low  temperature 
with  high  soda  excess,  the  formation  of  speiss  may  be  prevented. 

ANTIMONY.  —  This  metal  is  frequently  found  associated  with 
lead,  usually,  however,  only  in  small  amounts.  In  the  assay  with 
iron,  antimony  is  reduced  and  passes  into  the  lead  button.  But- 
tons containing  antimony  are  harder  and  whiter  than  those  from 


244  A   TEXTBOOK  OF  FIRE  ASSAYING 

pure  lead  ores  and  when  they  contain  much  antimony  are  brittle, 
breaking  with  a  bright  crystalline  fracture. 

If  much  antimony  is  present  (over  half  as  much  as  the  lead) 
an  antimony  speiss  will  be  found  lying  on  top  of  the  button. 

BISMUTH.  —  This  metal  is  rarely  found  associated  with  lead  ores, 
but  if  present  will  be  reduced  and  pass  into  the  lead  buttons. 

COPPER.  —  Copper  is  often  found  in  lead  ores  in  the  form  of 
chalcopyrite,  chalcocite,  and  oxidized  copper  compounds.  If  the 
ore  is  fully  oxidized  and  a  high  temperature  is  employed  most  of 
the  copper  will  pass  into  the  lead  button.  If  the  ore  contains 
much  pyrite,  or  sulphur  in  other  forms  most  of  the  copper  will 
remain  as  a  sulphide  and  be  dissolved  in  the  alkaline  slag.  A 
button  containing  copper  will  be  hard  and  tough  and  may  show 
a  reddish  tinge. 

IRON.  —  This  metal  is  often  present  in  lead  ores,  usually  in  the 
form  of  iron  pyrite.  It  goes  into  the  slag,  forming  either  a  silicate 
or  a  double  sulphide  of  iron  with  sodium  or  potassium.  The  lead 
button  is  practically  free  from  iron. 

ZINC.  —  Zinc  is  often  found  associated  with  lead  in  ores,  usu- 
ally in  the  form  of  the  sulphide.  During  the  assay,  part  of  the 
zinc  is  volatilized  and  part  remains  in  the  slag.  Zinc  sulphide  is 
only  decomposed  by  iron  at  a  very  high  temperature,  so  that  only 
a  very  small  amount  of  zinc  passes  into  the  lead  button.  Zinc 
sulphide  is  practically  infusible;  it  makes  the  slag  thick  and  pasty, 
and  thus,  if  present  in  too  great  proportion,  interferes  with  the 
separation  of  the  lead. 

Procedure.  —  Assay  ores  in  duplicate,  using  10  grams  of  ore 
and  40  grams  of  prepared  lead  flux.  Use  a  12-  or  15-gram  muffle 
crucible.  Weigh  out  first  30  grams  of  lead  flux,  place  the  ore  on 
top  of  this  and  mix  thoroughly  with  the  spatula.  Insert  a  spike 
or  nails,  point  downward,  and  finally  cover  with  10  grams  more  of 
lead  flux.  Have  the  muffle  just  visibly  red  and  bring  up  the  heat 
very  gradually  so  that  after  the  charges  are  put  in  it  will  take  at 
least,  forty-five  minutes  to  boil  them  down.  Close  the  door  of 
the  muffle  as  soon  as  the  crucibles  are  in,  and  after  the  charges 
are  melted  place  two  crucibles  partly  full  of  soft  coal  in  the  mouth 
of  the  muffle  just  inside  of  the  door,  which  should  be  kept  as  tightly 
closed  as  possible.  Raise  the  temperature  gradually  to  a  bright 
yellow  and  continue  at  this  temperature  until  the  nails  can  be 
removed  free  from  lead. 


THE  LEAD  ASSAY  245 

Finally  take  the  crucibles  from  the  muffle,  using  a  pair  of 
muffle-crucible  tongs,  and  without  setting  them  down  quickly 
remove  the  nails  with  a  large  pair  of  steel  forceps,  tapping  against 
the  side  of  the  crucible  and  washing  the  nails  in  the  slag  to  remove 
all  adhering  lead  globules.  Then  pour  into  a  deep,  pointed  mold. 
Work  as  fast  as  possible  to  prevent  too  great  chilling  of  the  slag 
in  the  crucible  before  pouring. 

When  cool  separate  the  lead  from  the  slag  and  hammer  clean. 
Weigh  to  centigrams  and  report  the  results  in  percentage.  Dupli- 
cates should  check  within  0.2  per  cent. 

The  slag  should  be  black  and  glassy.  If  it  is  dull,  more  borax- 
glass  should  be  added.  It  should  pour  well  from  the  crucible 
and  immediately  after  it  is  poured,  the  crucible  should  be  exam- 
ined for  shots  of  lead.  If  these  are  found  it  is  usually  an  indication 
of  too  low  a  temperature  at  pouring. 

Notes:  1.  If  the  ore  is  an  oxide  and  contains  copper  add  a  gram  or 
two  of  finely  pulverized  sulphur  to  the  charge  to  prevent  the  copper  from 
entering  the  button. 

2.  The  soft  coal  is  added  to  ensure  reducing  conditions  in  the  muffle  and  it 
may  be  renewed  if  necessary.     When  a  muffle  is  used  solely  for  fusion  purposes 
the  hole  in  the  back  is  stopped  up,  preventing  the  entrance  of  so  much  air. 

3.  The  removal  of  nails    and    the    pouring    must   be    done    without    a 
moment's  delay  as  the  charges  are  small  and  cool  rapidly. 

4.  If  the  ore  contains  much   silver  the  button  should  be  cupeled  and 
the  weight  of  silver  found  deducted 

5.  The  lead  should  be  soft  and  malleable  and  a  fresh  cut  surface  should 
have  the  bluish-gray  color  of  pure  lead.     The  button  should  be  capable  of 
being  hammered  out  into  a  thin  sheet  without  breaking  or  cracking.     A 
button  that  is  bright,  brittle  and  brilliantly  white  in  the  fracture  indicates 
the  presence  of  antimony. 

6.  The  lead  button  should  be  carefully  examined  for  speiss  before  it  is 
hammered.     With  a  little  care  this  may  be  pounded  off  without  seriously 
affecting  the  weight  of  lead. 

7.  If  there  is  doubt  regarding  the  purity  of  the  lead  button  it  may  be 
tested  by  cupellation.     The  only  metals,  except  lead,  likely  to  be  present  are 
gold,  silver,  antimony,  copper  and  possibly  bismuth;    each  of  these  gives 
characteristic  indications  in  cupeling. 

8.  Crucibles  may  be  used  a  number   of   times   as   they   are    but  little 
corroded,  but  those  used  previously  for  gold  and  silver  assays  must  not  be 
used  for  this  assay  as  the  slag  left  in  them  contains  lead.     It  is  well  to  use  a 
special  size  of  crucible  for  the  lead  assay  in  order  to  prevent  errors  due  to 
mixing  crucibles. 

9.  If  the  fusion  has  been   properly  conducted  the  nails  will  show  but 
little  corrosion.     If  they  are  much  corroded  the  results  are  bound  to  be  de- 
cidedly low. 


246  A    TEXTBOOK  OF  FIRE  ASSAYING 

Assay  of  Slags,  Furnace  Products  and  Low-grade  Ores  or 
Tailings.  —  In  the  assay  of  low-grade  materials,  such  as  slags  and 
tailings,  a  larger  quantity  of  ore  and  a  different  mixture  of  fluxes 
should  be  used.  The  slag  should  be  between  a  singulo-  and  a 
sub-silicate  and  part  of  the  iron  may  be  added  in  the  form  of 
filings.  On  account  of  the  size  of  the  charge  it  is  well  to  add  a 
number  of  nails,  as  this  will  lessen  the  time  necessary  for  complete 
reduction. 

The  following  charges  have  been  found  satisfactory: 

Limestone  (|-2  per  cent  Pb)       Slag  Slag 

Ore  25  grams  Slag  25  grams  Slag  100  grams 

Na2CO3         25       "      Na2CO3         25       "  Na2CO3        50     " 

K2CO3          20      "      K2CO3  20       "  K2CO3 

Borax-glass  20       "      Borax-glass  10       "  Borax-glass  10      " 

Flour  10       "      *Flour  10       "  Flour  10      " 

Nails  5      "      Nails  5       "  Nails  5      " 

(20-penny)  (20-penny)  (20-penny) 

20-gram  crucible  20-gram  crucible         30-gram  crucible 

Allow  some  time  at  a  high  temperature,  so  that  all  of  the  slag 
may  have  a  chance  to-  come  in  contact  with  the  iron. 

Corrected  Lead  Assay.  —  To  recover  any  lead  which  may  have 
been  left  in  the  slag  the  following  procedure  is  recommended: 
Save  all  the  slag  and  remelt  in  the  original  crucible  with  the  spikes 
or  nails  formerly  used.  If  the  first  slag  was  quite  glassy  and  vis- 
cous in  pouring,  add  from  5  to  15  grams  more  of  sodium  carbonate. 
Heat  to  redness  and  drop  into  each  crucible  a  lump  of  about  5 
grams  of  potassium  cyanide.  Close  the  door  of  the  muffle,  heat . 
to  a  bright  yellow  and  pour  as  soon  as  quiet.  Add  the  weight  of 
any  small  button  found  to  the  lead  from  the  original  fusion. 

Chemical  Reactions  of  the  Lead  Assay.  —  With  an  ore  con- 
taining PbC03,  PbSO4,  PbS,  SiO2  and  CaCO3  the  following  reac- 
tions may  occur: 

PbCO3  =  PbO  +  C02,  (Begins  at  200°  C.) 

2PbO  +  C  =  2Pb  +  CO2,  (Begins  at  550°  C.) 

PbO  +  Si02  =PbSi03,  (Begins  at  625°  C.) 

PbSO4  +  2C  =  PbS  +  2CO2,  (Begins  at  a  dark  red  heat.) 

7PbS  +  4K2CO3  =  4Pb  +  3(K2PbS2)  +  K2SO4  +  4C02. 

(Begins  at  a  red  heat.) 


THE  LEAD  ASSAY  247 

If  carbon  were  not  present  some  oxide  and  sulphate  would 
probably  remain  to  react  as  follows: 

PbS  +  2PbO  =  3Pb  +  SO2,  (Begins  at  720°  C.) 

PbS  +  PbSO4  =  2Pb  +  2SO2,  (Begins  at  670°  C.) 

2PbSO4  +  SiO2  =  Pb2SiO4  +  2SO2  +  O2.    (High  heat.) 

Toward  the  end,  as  the  heat  is  raised  to  a  bright  red  and  above, 
the  reactions  with  iron  become  important,  particularly  the  follow- 
ing: 

PbS  +  Fe  =  Pb  +  FeS, 

PbSiO3  +  Fe  =  Pb  +  FeSiO3,   (Requires  a  bright  yellow  heat  for 

completion.) 
K2PbS2  +  Fe  =  Pb  +  K2FeS2.  (Requires  a  bright  yellow  heat  for 

completion.) 


INDEX 


Active  flux,  definition,  171. 
Alumina,  fluxing  of,  168-170. 
Annealing,  120. 

reasons  for,  120. 
Annealing  cups,   clay  dish  to  hold, 

38. 

Antimony,  assay  of  ores  high  in,  205, 
206. 

behavior  in  cupellation,  110. 

behavior  hi  iron  nail  assay,   189, 
190. 

behavior  hi  scorification,  128,  137. 

effect  in  lead  assay,  243. 
Argols,  11. 

Arsenic,     behavior    in    cupellation, 
111. 

behavior  in  iron  nail  assay,  189. 

behavior  in  scorification,  128,  137. 

effect  in  lead  assay,  244. 
Assay-ton  weights,  84,  86. 

Balance,   alignment  of  knife  edges, 
testing,  84. 

assay,  73-75 

arms,  equality,  testing,  84. 

construction  of,  73,  74. 

directions  for  use  of,  77,  78. 

equilibrium,  testing,  82. 

flux,  71,  72. 

multiple  rider  attachment  for,  85, 
86. 

pulp,  72,  73. 

resistence,  testing,  82,  83. 

sensitivity,  73,  83,  84. 

stability,  73,  82. 

testing,  81-84. 

theory  of,  75,  77. 

time  of  oscillation,  73,  82. 
Basic  ores,  assay  of,  164-170. 


Basic  ores,  calculation  of  charge  for. 
166. 

slags  for,  164,  165. 

Bismuth,    behavior    in    cupellation, 
111,  115. 

behavior  in  scorification,  128. 

effect  in  lead  assay,  244. 

in  ores  from  Cobalt,  198. 
Bone-ash,  89-90. 

best  size  for  cupels,  90. 

fluxing  of,  209. 

specifications  for,  90. 

temperature  of  burning,  influence 
of,  89. 

to  preserve  muffles,  28. 
Bone-ash  cupel,  assay  of,  209. 
Borates,  classification  of,  4,  5. 
Borax,  3-6. 

action  in  slags,  148-150,  152. 

cover,  152,  172. 

effect   on   elimination   of   copper, 
202. 

quantity  required,  164,  165,   172, 

185. 

Borax  glass,  4-6. 
Bullion,  211. 

copper,  assay  of,  220-22.6. 

dore,  assay  of,  226-229. 

gold,  assay  of,  229-232. 

lead,  assay  of,  99,  219. 

sampling  of,  212-219. 

segregation    of    metals    in,    212- 
216. 

silver,  assay  of,  226.  , 

Capsules,  parting,  38,  119. 
Character  of  sample,  determination 

of,  145,  146. 
Charcoal,  11. 


249 


250 


INDEX 


Chiddey,  method  for  assay  of  cyan- 
ide solutions,  235,  236. 
Class    1   ores,   assay  procedure  for, 

170-173. 

slags  for,  162-165. 
Class   2   ores,  assay   procedure   for, 

186-188,  192-194. 
various  methods  of  assaying,  173- 

174. 
Class  3  ores,   assay  procedure  for, 

195. 

Clay,  fluxing  of,  168-170. 
Coal  furnaces,  18-23. 

firing  of,  23. 
Cobalt,    assay    of    ores    containing, 

196-198. 

behavior  in  scorification,  128. 
Coke  furnaces,  23,  24. 
Colorimetric  assay  of  solutions,  239. 
Combination  method  of  assay,  174, 

194. 

for  copper  bullion,  222-226. 
.  for    ores    containing    nickel    and 

cobalt,  197,  198. 
for  zinc-box  precipitate,  205. 
Copper,  assay  of  ores  high  in,  201- 

204. 

behavior  in  cupellation,  111,  115. 
behavior  in  scorification,  128. 
color  of  crucible  slags  containing, 

147. 
color  of  scorifier  slags  containing, 

137. 
effect  in  cupellation,  98,  105-107, 

232. 

effect  in  iron  nail  assay,  189-192. 
effect  in  lead  assay,  244. 
matte,  assay  of,  139,  201-204. 
Copper  bullion,  assay  of,  220-226. 
sampling  of,  217-218. 
segregation    of    metals    in,     213, 

214. 
Corrected  assays,  107-109,  115,  125, 

140,  205,  206-210,  219,  246. 
Cover,  the,  152,  153. 
Cream  of  tartar,  11. 
Crucible  assay,  theory  of,  143,  178- 
180,  182-184,  187. 


Crucibles,  31-33. 

capacity  of  different  sizes,  33. 
desirable  properties  of,  31,  32. 
sizes  for  various  charges,  203. 
testing  of,  32. 
Cryolite,  14,  170. 
Cupels,  89-93,  100. 
assay  of,  209,  210. 
cracking  of,  90,  111. 
effect  of  shape  of,  92,  93. 
instructions  for  making,  91,  92. 
machines  for  making,  91,  92. 
magnesia,  116,  117 
Portland  cement,  115,  116. 
size  of,  93. 

specifications  for,  92. 
testing,  109. 
Cupel  tray,  37,  38. 
Cupellation,  89 

assay  of  lead  bullion,  99,  219. 
correct  temperature  for,  94-97. 
description  of  process,  93-97. 
flashing  of  beads  from,  95. 
freezing  of  assay  during,  94,  95. 
indications  of  metals  present,  96, 

99,  109-114. 
instructions  for,  97-99. 
loss  of  gold  in,  102-105. 
loss    of    silver    in,    90,     100-102, 

116. 
regulation  of  temperature  during, 

94,  95,  97,  98. 
retention  of  base  metals  hi  beads 

from,  96,  98,  114,  115. 
spitting  during,  92,  326. 
sprouting  of  beads  from,  96,  98, 

226,  228. 

Cupellation  losses,  90,  99-109. 
influence  of  copper  on,  105-107. 
influence   of    impurities   on,    105- 

107. 
influence  of  quantity  of  lead  on, 

101,  102. 

influence  of  tellurium  on,  199,  200. 
influence  of  temperature  on,   100, 

102. 

effect  of  silver  on  gold,  103-105. 
progressive,  101. 


INDEX 


251 


Cupellation    losses,    rule    governing, 

107-109. 

Sharwood's  rule  for  determining, 
107-109. 

Desulphurizing  agent,  2. 
Dore  bullion,  assay  of,  226-229. 
sampling  of,  218. 

Electrolytic  assay  of  cyanide  solu- 
tions, 238. 

Ferric  oxide,  fluxing  of,  168 

oxidizing  effect  of,  157. 
Fire-brick,  directions  for  laying,  29. 
Fire-brick  lining  vs.  tile  lining,  18. 
Flour,  11. 

Fluorspar,  13,  170,  209. 
Fluxes  and  reagents,  1-14. 
Fluxing,  3,  5,  7,  9. 

principles  of,  2. 
Fuel,  17,  18. 
Fuel-oil  furnaces,  26-28. 
Furnace  repairs.  28-30. 
Furnaces,  16-28. 

directions  for  firing  soft  coal,  23. 
Fusion  products,  14,  15. 

'Gas  furnaces,  26. 

Gasoline  furnaces,  24-26. 

Glass,  3. 

Gold,  weighing  of,  78,  81,  120. 

Gold  bullion,  assay  of,  229-232. 

sampling  of,  218,  219. 
Gold  ores  containing  coarse  particles, 

assay  of,  66-70. 
Granulated  lead,  assay  of,  138. 
Grinder  for  assay  samples,  63-65. 

Inquartation,  121. 

Iridium,    behavior    during    cupella- 

tion,  113, 114. 
indications    of    in    appearance    of 

bead,  113. 

indications  of  in  parted  gold,  124. 
Iron,  11,  12,  188. 

behavior  in  cupellation,  110. 
behavior  in  scorification,  128. 


Iron,  color  of  crucible  slag,  contain- 
ing. 147. 

color    of    scorifier    slags,   contain- 
ing, 137. 

effect  in  lead  assay,  244. 

reducing  action  of ,  11,  12,  189. 
Iron  nail  assay,  173.  188-193. 

chemical    reactions    during,    189- 
191. 

limitations  of ,  191,  192. 

procedure  for,  192,  193, 

slag  for,  192, 

Lead,  11. 

fire  assay  of  ores,  240-247, 

granulated,  assay  of,  138. 

granulated  to  make,  11- 

ores,  classification  of,  240. 
Lead  assay,  accuracy  of,  240, 

assay  of  slags  from,  246. 

chemical    reactions    during,    246, 
247. 

conduct  of  fusion,  242. 

corrected,  246. 

influence  of  other  metals-on,  241, 
243,  244. 

losses  in,  241,  242, 

procedure  for,  244,  245, 

slag  for,  241..  242,  245. 
Lead  bullion,  assay  of,  99,  219. 

sampling  of,  216,  217, 
Lead  button,  14,  151,  152. 

testing  purity  of,  from  lead  assay, 

245. 

Limestone,  fluxing  of,  166-168, 
Litharge,  9-11, 

assay  of,  159-161. 

corrosive  action  of,  28,  133. 

disadvantages  of  excess,  176. 

quantity  required  to  slag  copper, 
202. 

solubility   of    metallic   oxides    in, 
127,  128. 

use  in  scorification  assay,  133. 

Magnesium    carbonate,    fluxing    of, 

167. 
Magnesium  oxide,  fluxing  of,  210. 


252 


INDEX 


Manganese,  dioxide,  oxidizing  effect 

of,  157. 

Manganese  oxide,  fluxing  of,  168. 
Matte,  15,  173. 

crucible  assay  of,  201-204. 

obtained    in   crucible   assay,    173, 
177,  180,  187,  193. 

seorification  assay  of,  139—141. 
Metallic  assay,  66-70. 
Metallic   oxidesr  heat  of  formation 
of,  128. 

solubility  in  litharge,  127,  128, 
Metallic   sulphides,  heat  of    forma- 
tion of,  190. 

ignition  temperature,  of,  128,  129. 
Minerals,  oxidizing  power  of,  157. 

reducing  power  of,  156. 
Moisture  sample,  60,  61. 
Mold,  crucible,  37. 

scorifier,  38. 
Muffles,  ai. 

care  of,  28. 

directions  for  replacing,  29,  30. 

methods  of  supporting,  22. 
Multiple  rider  attachment  for  bal- 
ances, 85, 86. 

Nickel,    assay    of    ores    containing,. 

196-198. 

behavior  in  cupellation,  1 10. 
behavior  in  seorification,  128,  130> 

137. 

effect  in  iron  nail  assay,  1 92V 
Niter  assay,  173-188. 

calculation  of  charge,  182-186, 
chemical  reactions  during,  159, 178— 

180,  182-184,  187. 
conduct  of  fusion,  176-178. 
disadvantages    of    excess    litharge 

in,  176, 
preliminary      fusion,      procedure, 

180-181. 
regular    fusion^    procedure,     186- 

188, 

slags  for  impure  ores,  175,  176, 
slags  for  pure  ores,  175. 
Niter,   determining  oxidizing  power 

of,  182. 


Niter,  oxidizing  power  of,  158,  I59\ 

quantity  required,  184. 

see  also  potassium  nitrate. 
Nitric   acid,    testing  for   impurities, 
125,  126. 

Oil  furnaces,  26-28. 
Ore,  classification  of,  1,  144,  240. 
determining    oxidizing    power    of, 

195. 
determining    reducing    power    of, 

180,  181. 
estimating  reducing  power  of,  18  lr 

182. 

in  general,  1. 

reducing  power  of  minerals,  156. 
Osmium,    behavior    during    cupella- 
tion, 113,  114. 

behavior  during  parting,  124. 
Oxides   metallic,   heat  of  formation 

of,  128. 

solubility  in  litharge,  127. 
Oxidizing  agent,  2. 
Oxidizing  power,  definition  of,  153. 
of  minerals,  157. 
of  niter,  158,  159. 
of  ores,  determination  of,  195. 
of  red  leadr  159. 

Oxidizing   reactions,    156-159,    178- 
180. 

Palladium,  behavior  during  parting, 

124. 
indications    of   in    appearance    of 

bead,  113, 

Indications  of  in  parting,  124. 
Parting,  118. 

acids  for,  118,  119, 

in  assay  of  gold  bullion,  231. 

in  porcelain  capsules,  119-121. 

errors    resulting    from,    123,    124',, 

125. 

in  flasks,  122,  123, 
disintegration  of  gold  during,  123', 

124,  125. 

indications  of  rare  metals  in,  124. 
influence  of  base  metals  on,  123V 

124, 


INDEX 


253 


Parting,    preparing  beads   for,    116, 

122. 

procedure,  119-121. 
recovery  of  gold  lost  in,  125. 
ratio  of  silver  to  gold  necessary, 

121. 
testing  for  completeness  of,    120, 

121. 
Platinum,   behavior  during  cupella- 

tion,  112,  114. 

behavior  during  parting,  124. 
indications    of    in    appearance    of 

bead,  112. 

indications  of  in  parted  gold,  124. 
Portland  cement,  flux  for,  210. 
Potassium  carbonate,  8,  9. 
Potassium  cyanide,  13. 
Potassium  nitrate,  12. 
Pulverizer  disc,  63-65, 

Reagents,  1-14. 

testing  of,  159-161. 
Red  lead,  oxidizing  power  of,  159. 
Reducing  agent,  2. 
Reducing  power,  definition  of,  153. 

of  minerals,  144,  155,  156. 

of    ores,    determination    of,    180, 
181. 

of  ores,  estimation  of,  181,  182. 

of  reagents,  154,  161. 

of  reagents,  determination  of,  160. 
Reducing  reactions,  153-156. 
Rhodium,   behavior  during  cupella- 
tion,  113,  114. 

indications    of    in    appearance    of 

bead,  112. 
Riders,  84,  85. 

multiple  attachment  for,  85,  86. 

testing,  88. 

Thompson,  85. 
Riffle  sampler,  51-54. 
Roasting,  193,  194. 

period  in  scorification  assay,  131. 

reactions  in  scorification,  135. 
Roasting  method  of  assay,  174,  193, 

194. 

Ruthenium,  behavior  during  cupel- 
lation,  114. 


Ruthenium,     indications    of    in    ap- 
pearance of  bead,  113. 

Salt,  13. 

cover,  152,  153. 
Sample,  definition,  39. 
finishing  the,  62-66. 
grab,  60. 
moisture,  60,  61. 
Sampler,    Brunton,    52,    53,    55-57, 

59. 

Jones,  53. 
Snyder,  57,  58, 
Vezan,  50-58. 
Sampling,    Brun ton's    formula    for, 

44-46. 

bullion,  212-216. 
copper  bullion,  217,  218. 
dore  bullion,  218. 
duplicate,  62. 
gold  bullion,  218,  219. 
grab,  60. 
hand,  48-54. 
lead  bullion,  216,  217. 
machine,  54-59, 
methods,  40,  41. 
mill,  complete,  59. 
moisture,  60,  61. 
object  of,  39, 
ore,  39-70. 
ores  containing  malleable  minerals, 

66-70. 

practice,  47-62. 
principles,  42-47. 
Richard's  rule  for,  43. 
tables  showing  weights  to  be  taken, 

43,  46. 
Scorification,  129. 

chemical    reactions    during,    134- 

136. 
effect   of   various   constituents   of 

ore  on,  133. 
indications  of  metals  present,  131, 

136,  137. 

losses  in,  139-141. 
ores  suited,  133,  134. 
reagents  used,  127. 
spitting  during,  137,  138,  139. 


254 


INDEX 


Scorification  assay,  127. 

charges  few  different  ores,  142. 

for  gold,  138,  220,  221. 

fractional  elimination  of  metals  in^ 
128,  129. 

of  copper  bullion,  220r  221. 

of  copper  matte,  139. 

procedure  for,  130-133. 

use  of  large  ore  charges  in,  141. 
Scorifiers,  33,  34. 

sizesr  33,  127. 

Screening  assay  samples,  65. 
Segregation    of    metals    in    cooling, 
influence  of  on  sampling,  212- 
216. 

Silica,  2,  3. 
Silicates,  classification  of,  147,  148, 

mixed,  151. 
Siliceous  ores,  calculation  of  charge 

for,  162-164. 

Silver  foil,  testing  for  gold,  126, 
Slags,  14,  146. 

acid  and  basic  distinguished,  151. 

action  of  borax  in,  143,  148-150, 

assay  of,  208,  246. 

color  of  crucible,  147, 

color  of  seorifier.  136,  137. 

for  class  I  basic  ores,  164-168, 

for  class  1  siliceous  ores,  162-164. 

for  class  2.  iron  assay,  192. 

for  class  2,  niter  assay,  175,  176. 

for  crucible  assay  of  ores  contain- 
ing copper,  202. 

fluidity  of,  150. 

formation  temperature  of,  14& 

properties  of  good,  146,  147. 
Slag  factors,  bisilicate,  163,  165X 

mono-silicate,  188. 
Sodium  bicarbonate,  6. 
Sodium  carbonate,  6-8. 
Solutions,  assay  of,  233-239 
Speiss,  15. 

in  crucible  assay,  189. 

in  lead  assay,  243. 
Split  shovel,  51-53. 
Splitter,  sample,  51-54. 
Sprouting,  to  prevent,  22S 
Statk,  height  of,  22, 


Stack,  support  of,  22. 
vSulphides,    heats    of    formation    of 
metallic,  190. 

ignition  temperatures  of,  128,  129, 

reactions  with  iron,  189-191. 

reactions  with  niter,  169,  183. 

reactions  with  oxides,  136. 

reactions  with  oxygen,  135. 

reducing  powers  of,  I55r  156. 

Telluride  ores,  assay  of,  198-201. 
Tellurium,   behavior  in   eupellation,, 
111,  199,  200. 

behavior  in  crucible  fusions,  200,, 
201, 

behavior  in  scorification,  128. 

indications  of,  in  beads,  99,  111. 
Temperature,  eye  estimation  of,  117- 
Tin,  assay  of  ores  high  in,  206. 

behavior  in  eupellation,  110 
Tools,  furnace,  34-37. 

Vanning,  operation  of,  145,  146. 

Weighing,  accumulative,  81. 

assay  pulp,  directions  for,  132- 

by  equal  swings,  78,  79. 

by  methods  of  swings,  79,  80i 

by  "no  deflection,"  80. 

by  substitution,  80,  81. 

check,  81. 

double,  77. 

general  directions  for,  77,  78. 

gold,  78,  120. 

silver,  78,  99. 
Weights,  84-86. 

assay-ton,  84,  86. 

calibration  of,  86-88, 

milligram,  84, 

millieme,  211. 

recording,  78. 
Wood  fumaeesy  23. 

Zincy  behavior  in  eupellation,  1IO\ 
effect  in  iron  nail  assay,  190. 
effect  in  lead  assay,  244. 

Zinc-box  precipitate,  assay  of,  204, 
205. 


GENERAL  LIBRARY 
UNIVERSITY  OF  CALIFORNIA— BERKELEY 

RETURN  TO  DESK  FROM  WHICH  BORROWED 

This  book  is  due  on  the  last  date  stamped  below,  or  on  the 

date  to  which  renewed. 
Renewed  books  are  subject  to  immediate  recall. 


HOV 1 5  1954  J 


NO1 


LD  21-lOOm-l, '54(1887sl6)476 


0740 


INTERNATIONAL  ATOMIC  WEIGHTS  1921    (PARTIAL) 


Element 

Sym- 
bol 

Atomic 
Weight 

Element 

Sym- 
bol 

Atomic 
Weight 

Aluminum 

Al 

27.1 

Molybdenum.  .  . 

Mo 

96  0 

Antimony  

Sb 

120.2 

Nickel  

'   Ni 

58.68 

J  (Arsenic    

As 

74.96 

Nitrogen  

N 

14.008 

Barium 

Ba 

137  37 

Osmium 

Os 

190  9 

Bismuth 

Bi 

208  0 

Oxygen 

o 

16  00 

Boron 

B 

10  9 

Palladium 

Pd 

106.7 

Bromine 

Br 

79  92 

Phosphorus  .... 

P 

31  04 

Cd 

120  40 

Platinum  

Pt 

195  2 

Calcium    

Ca 

40  07 

Potassium  

K 

39  10 

Carbon  

c 

12.005 

Rhodium  

Rh 

102.9 

Chlorine  
Chromium  

Cl 
Cr 

35.46 
52.0 

Ruthenium  
Selenium  ...... 

Ru 

,  Se 

101.7 
79.2 

Cobalt. 

Co 

58  97 

Silicon  ^          ..» 

tei 

28  3 

Copper 

Cu 

63  57 

Silver 

Ag 

107  88 

Fluorine 

F 

19  0 

Sodium 

Na 

23  00 

Gold  

Au 

197  2 

Strontium 

Sr 

87.63 

Hydrogen  

H 

1  008 

Sulphur  

s 

32.06 

Iodine  

I 

126  92 

Tellurium  

Te 

127.5 

Iridium. 

Ir 

193  1 

Tin 

Sn 

118  7 

Iron  

Fe 

55  84 

Titanium  

Ti 

48.1 

Lead  .   . 

Pb 

207  20 

Tungsten 

W 

184  0 

Lithium 

Li 

6  94 

Uranium  . 

U 

238  2 

Magnesium 

Mg 

24  32 

Vanadium    .  .    . 

V 

51  0 

Manganese  .  . 

Mn 

54  93 

Zinc    

Zn 

65.37 

Mercury  

He 

200  6 

Zirconium  

Zr 

90.6 

